diff --git a/node/test_utxo_db.py b/node/test_utxo_db.py
index 78352f80..355de931 100644
--- a/node/test_utxo_db.py
+++ b/node/test_utxo_db.py
@@ -343,6 +343,62 @@ def test_mining_reward_empty_inputs_allowed(self):
         self.assertTrue(ok)
         self.assertEqual(self.db.get_balance('alice'), 100 * UNIT)
 
+    # -- bounty #2819 LOW: validation gaps & edge cases ----------------------
+
+    def test_duplicate_input_rejected(self):
+        """Same box_id listed twice in inputs must be rejected.
+        Without explicit dedup, input_total is inflated 2x (LOW-2)."""
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+        box_id = boxes[0]['box_id']
+
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [
+                {'box_id': box_id, 'spending_proof': 'sig'},
+                {'box_id': box_id, 'spending_proof': 'sig'},  # duplicate
+            ],
+            'outputs': [{'address': 'attacker', 'value_nrtc': 200 * UNIT}],
+            'fee_nrtc': 0,
+        }, block_height=10)
+        self.assertFalse(ok)
+        self.assertEqual(self.db.get_balance('alice'), 100 * UNIT)
+        self.assertEqual(self.db.get_balance('attacker'), 0)
+
+    def test_self_transfer(self):
+        """Self-transfer (from == to) must work correctly."""
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [{'box_id': boxes[0]['box_id'],
+                         'spending_proof': 'sig'}],
+            'outputs': [{'address': 'alice', 'value_nrtc': 100 * UNIT}],
+            'fee_nrtc': 0,
+        }, block_height=10)
+        self.assertTrue(ok)
+        self.assertEqual(self.db.get_balance('alice'), 100 * UNIT)
+
+    def test_spending_proof_accepted_without_verification(self):
+        """The UTXO layer accepts any spending_proof without verification.
+        Signature verification is the endpoint layer's responsibility.
+        This test documents the behavior so future changes don't
+        accidentally rely on it (LOW-3)."""
+        self._apply_coinbase('alice', 100 * UNIT)
+        boxes = self.db.get_unspent_for_address('alice')
+
+        # Bogus spending_proof is accepted at the UTXO layer
+        ok = self.db.apply_transaction({
+            'tx_type': 'transfer',
+            'inputs': [{'box_id': boxes[0]['box_id'],
+                         'spending_proof': 'TOTALLY_BOGUS'}],
+            'outputs': [{'address': 'bob', 'value_nrtc': 100 * UNIT}],
+            'fee_nrtc': 0,
+        }, block_height=10)
+        self.assertTrue(ok, "UTXO layer should accept any spending_proof "
+                            "(verification is endpoint's job)")
+
     def test_mempool_empty_inputs_rejected_for_transfer(self):
         """Mempool must also reject non-minting txs with empty inputs."""
         tx = {
diff --git a/node/utxo_db.py b/node/utxo_db.py
index db77aaf1..65470e17 100644
--- a/node/utxo_db.py
+++ b/node/utxo_db.py
@@ -292,6 +292,12 @@ def apply_transaction(self, tx: dict, block_height: int,
         """
         Atomically apply a transaction: spend inputs, create outputs.
 
+        .. warning::
+            This method does **not** verify ``spending_proof``.  Callers
+            MUST authenticate the spender (e.g. Ed25519 signature check)
+            before calling this method.  See ``utxo_endpoints.py`` for
+            the endpoint-level verification.
+
         ``tx`` keys:
             tx_type: str
             inputs: list of {box_id: str, spending_proof: str}
@@ -316,6 +322,18 @@ def apply_transaction(self, tx: dict, block_height: int,
         try:
             conn.execute("BEGIN IMMEDIATE")
 
+            # -- reject duplicate input box_ids --------------------------------
+            # Keyed on box_id alone (the PK of the UTXO being consumed).
+            # Different spending_proof values for the same box_id are still
+            # a duplicate — the proof content is irrelevant to dedup.
+            # Without this, the same box_id counted twice inflates
+            # input_total.  The spend-phase rowcount check catches it
+            # today, but only accidentally.  Defense in depth.
+            input_box_ids = [i['box_id'] for i in inputs]
+            if len(input_box_ids) != len(set(input_box_ids)):
+                conn.execute("ROLLBACK")
+                return False
+
             # -- validate inputs exist and are unspent -----------------------
             input_total = 0
             for inp in inputs: