diff --git a/db/migrations/20260421120000_create_sizes_sizegroup_table.php b/db/migrations/20260421120000_create_sizes_sizegroup_table.php
new file mode 100644
index 000000000..12b5e5f63
--- /dev/null
+++ b/db/migrations/20260421120000_create_sizes_sizegroup_table.php
@@ -0,0 +1,86 @@
+<?php
+
+declare(strict_types=1);
+
+use Phinx\Migration\AbstractMigration;
+
+/**
+ * Creates the sizes_sizegroup cross-reference table that stores the many-to-many
+ * relationship between sizes and sizegroups, then drops the now-redundant
+ * sizegroup_id and seq columns from the sizes table.
+ *
+ * This migration runs BEFORE DeduplicateSizes so that every size – including
+ * the duplicates that will later be removed – already has a row in
+ * sizes_sizegroup before deduplication happens. When DeduplicateSizes removes
+ * duplicate rows from sizes, the corresponding sizes_sizegroup rows are removed
+ * automatically via the size_id foreign key's ON DELETE CASCADE behavior.
+ */
+final class CreateSizesSizegroupTable extends AbstractMigration
+{
+    public function up(): void
+    {
+        // ----------------------------------------------------------------
+        // 1. Create cross-reference table
+        // ----------------------------------------------------------------
+        $this->execute('CREATE TABLE `sizes_sizegroup` (
+            `size_id`      INT UNSIGNED NOT NULL,
+            `sizegroup_id` INT UNSIGNED NOT NULL,
+            `seq`          INT          DEFAULT NULL,
+            PRIMARY KEY (`size_id`, `sizegroup_id`),
+            KEY `idx_sizegroup_id` (`sizegroup_id`),
+            CONSTRAINT `fk_ss_size_id`
+                FOREIGN KEY (`size_id`)      REFERENCES `sizes`     (`id`)
+                ON DELETE CASCADE ON UPDATE CASCADE,
+            CONSTRAINT `fk_ss_sizegroup_id`
+                FOREIGN KEY (`sizegroup_id`) REFERENCES `sizegroup` (`id`)
+                ON DELETE CASCADE ON UPDATE CASCADE
+        ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC');
+
+        // ----------------------------------------------------------------
+        // 2. Populate from existing sizes.sizegroup_id (one row per size,
+        //    including rows for the duplicates that DeduplicateSizes will
+        //    later remove along with their sizes_sizegroup entries)
+        // ----------------------------------------------------------------
+        $this->execute('INSERT INTO `sizes_sizegroup` (`size_id`, `sizegroup_id`, `seq`)
+            SELECT `id`, `sizegroup_id`, `seq`
+            FROM   `sizes`
+            WHERE  `sizegroup_id` IS NOT NULL');
+
+        // ----------------------------------------------------------------
+        // 3. Drop the now-redundant columns from sizes
+        // ----------------------------------------------------------------
+        $this->table('sizes')->dropForeignKey('sizegroup_id')->save();
+        $this->table('sizes')->removeColumn('sizegroup_id')->save();
+        $this->table('sizes')->removeColumn('seq')->save();
+    }
+
+    public function down(): void
+    {
+        // Restore sizegroup_id and seq columns on sizes
+        $this->execute('ALTER TABLE `sizes`
+            ADD COLUMN `sizegroup_id` INT UNSIGNED DEFAULT NULL AFTER `label`,
+            ADD COLUMN `seq`          INT          DEFAULT NULL AFTER `sizegroup_id`,
+            ADD KEY    `sizegroup_id` (`sizegroup_id`),
+            ADD CONSTRAINT `sizes_ibfk_4`
+                FOREIGN KEY (`sizegroup_id`) REFERENCES `sizegroup` (`id`)
+                ON UPDATE CASCADE');
+
+        // Re-populate from the cross-reference table.
+        // Where a size belongs to multiple sizegroups, pick the row with the
+        // smallest sizegroup_id as the "primary" one (mirrors the original data).
+        $this->execute('UPDATE `sizes` s
+            JOIN (
+                SELECT   size_id, sizegroup_id, seq
+                FROM     sizes_sizegroup ss1
+                WHERE    sizegroup_id = (
+                    SELECT MIN(sizegroup_id)
+                    FROM   sizes_sizegroup ss2
+                    WHERE  ss2.size_id = ss1.size_id
+                )
+            ) best ON best.size_id = s.id
+            SET s.sizegroup_id = best.sizegroup_id,
+                s.seq          = best.seq');
+
+        $this->execute('DROP TABLE `sizes_sizegroup`');
+    }
+}
diff --git a/db/migrations/20260421130000_deduplicate_sizes.php b/db/migrations/20260421130000_deduplicate_sizes.php
new file mode 100644
index 000000000..020f56ce2
--- /dev/null
+++ b/db/migrations/20260421130000_deduplicate_sizes.php
@@ -0,0 +1,314 @@
+<?php
+
+declare(strict_types=1);
+
+use Phinx\Migration\AbstractMigration;
+
+/**
+ * Removes duplicate size labels from the sizes table (keeping the entry with the
+ * smallest ID for each label).  The preceding CreateSizesSizegroupTable migration
+ * has already created sizes_sizegroup and populated it with one row per size, so
+ * this migration remaps the sizes_sizegroup rows for duplicate sizes to their
+ * canonical ID (preserving sizegroup memberships), then deletes the duplicate sizes.
+ *
+ * All references in stock, shipment_detail, history, itemsout and distro_events_*
+ * tables are updated to point at the surviving size ID before the surplus rows
+ * are deleted.
+ *
+ * Removed ID → kept ID mapping (grouped by label):
+ *   S            53 → 1          M            54 → 2          L            55 → 3
+ *   34           67 → 28        35           65 → 29
+ *   39          57,186 → 33    40          58,187 → 34    41          59,188 → 35
+ *   Mixed      70,71,204-223 → 52
+ *   All ages   103,123,140,148,163 → 97
+ *   7-12 mo     120 → 47       13-18 mo    121 → 48       19-24 mo    122 → 44
+ *   6-10 yr     124 → 117      11-15 yr    125 → 118
+ *   2-5 yr      130 → 116      0-6 mo      138 → 119
+ *   2-3 yr      141 → 10       4-5 yr      142 → 11
+ *   24          171 → 18       25          172 → 19       26          173 → 20
+ *   27          174 → 21       28          175 → 22       29          176 → 23
+ *   30          177 → 24       31          178 → 25       32          179 → 26
+ *   33          180 → 27       34 (dup)    181 → 28       35 (dup)    182 → 29
+ *   36          183 → 30       37          184 → 31       38          185 → 32
+ *   39 (dup)    186 → 33       40 (dup)    187 → 34       41 (dup)    188 → 35
+ *   42          189 → 60       43          190 → 61       44          191 → 62
+ *   45          192 → 63
+ */
+final class DeduplicateSizes extends AbstractMigration
+{
+    private const REMOVED_IDS = '53,54,55,57,58,59,65,67,70,71,103,120,121,122,123,124,125,'
+        .'130,138,140,141,142,148,163,171,172,173,174,175,176,177,178,179,180,181,182,183,'
+        .'184,185,186,187,188,189,190,191,192,204,205,206,207,208,209,210,211,212,213,214,'
+        .'215,216,217,218,219,220,221,222,223';
+
+    /** SQL CASE expression mapping removed size IDs to their canonical replacement. */
+    private const REMAP_CASE = 'CASE size_id
+        WHEN  53 THEN   1  WHEN  54 THEN   2  WHEN  55 THEN   3
+        WHEN  57 THEN  33  WHEN  58 THEN  34  WHEN  59 THEN  35
+        WHEN  65 THEN  29  WHEN  67 THEN  28
+        WHEN  70 THEN  52  WHEN  71 THEN  52
+        WHEN 103 THEN  97  WHEN 123 THEN  97  WHEN 140 THEN  97
+        WHEN 148 THEN  97  WHEN 163 THEN  97
+        WHEN 120 THEN  47  WHEN 121 THEN  48  WHEN 122 THEN  44
+        WHEN 124 THEN 117  WHEN 125 THEN 118
+        WHEN 130 THEN 116  WHEN 138 THEN 119
+        WHEN 141 THEN  10  WHEN 142 THEN  11
+        WHEN 171 THEN  18  WHEN 172 THEN  19  WHEN 173 THEN  20
+        WHEN 174 THEN  21  WHEN 175 THEN  22  WHEN 176 THEN  23
+        WHEN 177 THEN  24  WHEN 178 THEN  25  WHEN 179 THEN  26
+        WHEN 180 THEN  27  WHEN 181 THEN  28  WHEN 182 THEN  29
+        WHEN 183 THEN  30  WHEN 184 THEN  31  WHEN 185 THEN  32
+        WHEN 186 THEN  33  WHEN 187 THEN  34  WHEN 188 THEN  35
+        WHEN 189 THEN  60  WHEN 190 THEN  61  WHEN 191 THEN  62
+        WHEN 192 THEN  63
+        WHEN 204 THEN  52  WHEN 205 THEN  52  WHEN 206 THEN  52
+        WHEN 207 THEN  52  WHEN 208 THEN  52  WHEN 209 THEN  52
+        WHEN 210 THEN  52  WHEN 211 THEN  52  WHEN 212 THEN  52
+        WHEN 213 THEN  52  WHEN 214 THEN  52  WHEN 215 THEN  52
+        WHEN 216 THEN  52  WHEN 217 THEN  52  WHEN 218 THEN  52
+        WHEN 219 THEN  52  WHEN 220 THEN  52  WHEN 221 THEN  52
+        WHEN 222 THEN  52  WHEN 223 THEN  52
+        ELSE size_id
+    END';
+
+    public function up(): void
+    {
+        $ids = self::REMOVED_IDS;
+        $case = self::REMAP_CASE;
+
+        // --- stock -------------------------------------------------------
+        $this->execute("UPDATE stock
+            SET size_id = {$case}
+            WHERE size_id IN ({$ids})");
+
+        // --- shipment_detail ---------------------------------------------
+        $sourceCaseExpr = str_replace('size_id', 'source_size_id', $case);
+        $targetCaseExpr = str_replace('size_id', 'target_size_id', $case);
+
+        $this->execute("UPDATE shipment_detail
+            SET source_size_id = {$sourceCaseExpr}
+            WHERE source_size_id IN ({$ids})");
+
+        $this->execute("UPDATE shipment_detail
+            SET target_size_id = {$targetCaseExpr}
+            WHERE target_size_id IN ({$ids})");
+
+        // --- history (size_id changes on stock rows) ---------------------
+        $fromCaseExpr = str_replace('size_id', 'from_int', $case);
+        $toCaseExpr = str_replace('size_id', 'to_int', $case);
+
+        $this->execute("UPDATE history
+            SET from_int = {$fromCaseExpr}
+            WHERE tablename = 'stock' AND changes = 'size_id'
+              AND from_int IN ({$ids})");
+
+        $this->execute("UPDATE history
+            SET to_int = {$toCaseExpr}
+            WHERE tablename = 'stock' AND changes = 'size_id'
+              AND to_int IN ({$ids})");
+
+        // --- itemsout ----------------------------------------------------
+        if ($this->hasTable('itemsout')) {
+            $this->execute("UPDATE itemsout
+                SET size_id = {$case}
+                WHERE size_id IN ({$ids})");
+        }
+
+        // --- distro_events_* tables with size_id -------------------------
+        foreach (['distro_events_packing_list_entries', 'distro_events_tracking_logs', 'distro_events_unboxed_item_collections'] as $dTable) {
+            if ($this->hasTable($dTable)) {
+                $this->execute("UPDATE `{$dTable}`
+                    SET size_id = {$case}
+                    WHERE size_id IN ({$ids})");
+            }
+        }
+
+        // --- sizes_sizegroup ---------------------------------------------
+        // Remap cross-reference rows: canonical sizes inherit all sizegroup
+        // memberships from their duplicates.  INSERT IGNORE handles the rare
+        // case where the canonical already belongs to the same sizegroup.
+        // The ON DELETE CASCADE on fk_ss_size_id then removes the old rows
+        // for the duplicate size IDs when they are deleted below.
+        $ssCaseExpr = str_replace('size_id', 'ss.size_id', $case);
+        $this->execute("INSERT IGNORE INTO `sizes_sizegroup` (`size_id`, `sizegroup_id`, `seq`)
+            SELECT {$ssCaseExpr}, ss.`sizegroup_id`, ss.`seq`
+            FROM `sizes_sizegroup` ss
+            WHERE ss.`size_id` IN ({$ids})");
+
+        // --- delete duplicate sizes (CASCADE removes their sizes_sizegroup rows) ---
+        $this->execute("DELETE FROM sizes WHERE id IN ({$ids})");
+    }
+
+    public function down(): void
+    {
+        // Re-insert the duplicate size rows that were removed by up().
+        // Note: sizegroup_id and seq columns no longer exist on sizes (they were
+        // dropped by the preceding CreateSizesSizegroupTable migration), so only
+        // id and label are restored here.  Foreign-key references in stock etc.
+        // that were remapped cannot be reconstructed.
+        $this->execute("INSERT IGNORE INTO sizes (id, label) VALUES
+            (53,'S'),
+            (54,'M'),
+            (55,'L'),
+            (57,'39'),
+            (58,'40'),
+            (59,'41'),
+            (65,'35'),
+            (67,'34'),
+            (70,'Mixed'),
+            (71,'Mixed'),
+            (103,'All ages'),
+            (120,'7-12 months'),
+            (121,'13-18 months'),
+            (122,'19-24 months'),
+            (123,'All ages'),
+            (124,'6-10 years'),
+            (125,'11-15 years'),
+            (130,'2-5 years'),
+            (138,'0-6 months'),
+            (140,'All ages'),
+            (141,'2-3 years'),
+            (142,'4-5 years'),
+            (148,'All ages'),
+            (163,'All ages'),
+            (171,'24'),
+            (172,'25'),
+            (173,'26'),
+            (174,'27'),
+            (175,'28'),
+            (176,'29'),
+            (177,'30'),
+            (178,'31'),
+            (179,'32'),
+            (180,'33'),
+            (181,'34'),
+            (182,'35'),
+            (183,'36'),
+            (184,'37'),
+            (185,'38'),
+            (186,'39'),
+            (187,'40'),
+            (188,'41'),
+            (189,'42'),
+            (190,'43'),
+            (191,'44'),
+            (192,'45'),
+            (204,'Mixed'),
+            (205,'Mixed'),
+            (206,'Mixed'),
+            (207,'Mixed'),
+            (208,'Mixed'),
+            (209,'Mixed'),
+            (210,'Mixed'),
+            (211,'Mixed'),
+            (212,'Mixed'),
+            (213,'Mixed'),
+            (214,'Mixed'),
+            (215,'Mixed'),
+            (216,'Mixed'),
+            (217,'Mixed'),
+            (218,'Mixed'),
+            (219,'Mixed'),
+            (220,'Mixed'),
+            (221,'Mixed'),
+            (222,'Mixed'),
+            (223,'Mixed')");
+
+        // Re-insert the sizes_sizegroup rows for the restored sizes
+        // (one row per size, matching the original sizes.sizegroup_id / sizes.seq values
+        // from init.sql that were inserted by CreateSizesSizegroupTable)
+        $this->execute('INSERT IGNORE INTO sizes_sizegroup (size_id, sizegroup_id, seq) VALUES
+            (53, 5, 50),
+            (54, 5, 51),
+            (55, 5, 52),
+            (57, 8, 81),
+            (58, 8, 82),
+            (59, 8, 83),
+            (65, 9, 118),
+            (67, 9, 117),
+            (70, 5, 53),
+            (71, 1, 6),
+            (103, 4, 205),
+            (120, 2, 41),
+            (121, 2, 42),
+            (122, 2, 43),
+            (123, 2, 44),
+            (124, 18, 20),
+            (125, 18, 21),
+            (130, 20, 30),
+            (138, 22, 180),
+            (140, 4, 182),
+            (141, 23, 200),
+            (142, 23, 201),
+            (148, 23, 204),
+            (163, 24, 234),
+            (171, 26, 251),
+            (172, 26, 252),
+            (173, 26, 253),
+            (174, 26, 254),
+            (175, 26, 255),
+            (176, 26, 256),
+            (177, 26, 257),
+            (178, 26, 258),
+            (179, 26, 259),
+            (180, 26, 260),
+            (181, 26, 261),
+            (182, 26, 262),
+            (183, 26, 263),
+            (184, 26, 264),
+            (185, 26, 265),
+            (186, 26, 266),
+            (187, 26, 267),
+            (188, 26, 268),
+            (189, 26, 269),
+            (190, 26, 270),
+            (191, 26, 271),
+            (192, 26, 272),
+            (204, 2, 999),
+            (205, 3, 999),
+            (206, 4, 999),
+            (207, 7, 999),
+            (208, 8, 999),
+            (209, 9, 999),
+            (210, 12, 999),
+            (211, 13, 999),
+            (212, 16, 999),
+            (213, 17, 999),
+            (214, 18, 999),
+            (215, 19, 999),
+            (216, 20, 999),
+            (217, 21, 999),
+            (218, 22, 999),
+            (219, 23, 999),
+            (220, 24, 999),
+            (221, 25, 999),
+            (222, 26, 999),
+            (223, 27, 999)');
+
+        // Remove the sizegroup memberships that canonical sizes inherited
+        // from their duplicates during up().  Each pair below is
+        // (canonical_size_id, sizegroup_id_of_the_duplicate).
+        $this->execute('DELETE FROM `sizes_sizegroup`
+            WHERE (`size_id`, `sizegroup_id`) IN (
+                ( 1,  5), ( 2,  5), ( 3,  5),
+                (33,  8), (34,  8), (35,  8),
+                (28,  9), (29,  9),
+                (52,  1), (52,  5),
+                (97,  2), (97,  4), (97, 23), (97, 24),
+                (44,  2), (47,  2), (48,  2),
+                (117, 18), (118, 18),
+                (116, 20),
+                (119, 22),
+                (10, 23), (11, 23),
+                (18, 26), (19, 26), (20, 26), (21, 26), (22, 26), (23, 26),
+                (24, 26), (25, 26), (26, 26), (27, 26),
+                (28, 26), (29, 26), (30, 26), (31, 26), (32, 26),
+                (33, 26), (34, 26), (35, 26),
+                (60, 26), (61, 26), (62, 26), (63, 26),
+                (52,  2), (52,  3), (52,  4), (52,  7), (52,  8), (52,  9),
+                (52, 12), (52, 13), (52, 16), (52, 17), (52, 18), (52, 19),
+                (52, 20), (52, 21), (52, 22), (52, 23), (52, 24), (52, 25),
+                (52, 26), (52, 27)
+            )');
+    }
+}
diff --git a/include/container-stock.php b/include/container-stock.php
index d6201c442..04374588f 100644
--- a/include/container-stock.php
+++ b/include/container-stock.php
@@ -28,8 +28,9 @@
                 s3.box_state_id = 1 AND 
                 l2.camp_id='.$_SESSION['camp']['id'].')-IFNULL(COUNT(s2.id),0) AS totalboxes
 	FROM
-		(products AS p,
-		sizes AS s)
+		products AS p
+	INNER JOIN sizes_sizegroup AS ss ON ss.sizegroup_id = p.sizegroup_id
+	INNER JOIN sizes AS s ON ss.size_id = s.id
 	LEFT OUTER JOIN genders AS g ON p.gender_id = g.id
 	LEFT OUTER JOIN 
         stock AS s2 
@@ -40,7 +41,6 @@
         s2.location_id IN ('.$container_stock_locations.')
 	WHERE
 		(NOT p.deleted OR p.deleted IS NULL) AND
-		s.sizegroup_id = p.sizegroup_id AND
 		p.camp_id = '.$_SESSION['camp']['id'].'
 		'.($_SESSION['search']['container-stock'] ? 'AND p.name LIKE "%'.$_SESSION['search']['container-stock'].'%"' : '').'
 	GROUP BY
diff --git a/include/stock_edit.php b/include/stock_edit.php
index 50890de0e..db2b47607 100644
--- a/include/stock_edit.php
+++ b/include/stock_edit.php
@@ -206,7 +206,7 @@
 
 addfield('select', 'Product', 'product_id', ['disabled' => $disabled, 'test_id' => 'product_id', 'required' => true, 'multiple' => false, 'query' => 'SELECT p.id AS value, CONCAT(p.name, " " ,IFNULL(g.label,"")) AS label FROM products AS p LEFT OUTER JOIN genders AS g ON p.gender_id = g.id WHERE (NOT p.deleted OR p.deleted IS NULL) AND p.camp_id = '.$_SESSION['camp']['id'].($_SESSION['camp']['separateshopandwhproducts'] ? ' AND NOT p.stockincontainer' : '').' ORDER BY name', 'onchange' => 'getSizes()']);
 
-addfield('select', 'Size', 'size_id', ['disabled' => $disabled, 'required' => true, 'width' => 2, 'multiple' => false, 'query' => 'SELECT *, id AS value FROM sizes WHERE sizegroup_id = '.intval(db_value('SELECT sizegroup_id FROM products WHERE id = :id', ['id' => $data['product_id']])).' ORDER BY seq', 'tooltip' => 'If the right size for your box is not here, don\'t put it in comments, but first double check if you have the right product. For example: Long sleeves for babies, we call them tops.']);
+addfield('select', 'Size', 'size_id', ['disabled' => $disabled, 'required' => true, 'width' => 2, 'multiple' => false, 'query' => 'SELECT s.id, s.label, s.id AS value FROM sizes AS s JOIN sizes_sizegroup AS ss ON ss.size_id = s.id WHERE ss.sizegroup_id = '.intval(db_value('SELECT sizegroup_id FROM products WHERE id = :id', ['id' => $data['product_id']])).' ORDER BY ss.seq', 'tooltip' => 'If the right size for your box is not here, don\'t put it in comments, but first double check if you have the right product. For example: Long sleeves for babies, we call them tops.']);
 
 addfield('number', 'Items', 'items', ['testid' => 'items_id', 'readonly' => $disabled]);
 
diff --git a/library/ajax/getsizes.php b/library/ajax/getsizes.php
index aae50f600..97327f99e 100644
--- a/library/ajax/getsizes.php
+++ b/library/ajax/getsizes.php
@@ -4,7 +4,7 @@
 $product_id = $_POST['product_id'];
 $size_id = intval($_GET['size']);
 
-$result = db_array('SELECT s.* FROM sizes AS s, sizegroup AS sg, products AS p WHERE s.sizegroup_id = sg.id AND p.id = :id AND p.sizegroup_id = sg.id ORDER BY s.seq', ['id' => $product_id]);
+$result = db_array('SELECT s.id, s.label FROM sizes AS s JOIN sizes_sizegroup AS ss ON ss.size_id = s.id JOIN products AS p ON p.sizegroup_id = ss.sizegroup_id AND p.id = :id ORDER BY ss.seq', ['id' => $product_id]);
 $html = '<option></option>';
 
 if (1 == count($result)) {