Fix DataGridViewComboBoxCell setting incorrect value when DisplayMember values are duplicated

src/System.Windows.Forms/System/Windows/Forms/Controls/DataGridView/DataGridViewComboBoxCell.cs

@@ -1580,6 +1580,24 @@ private bool LookupValue(object? formattedValue, out object? value)
         object? item;
         if (DisplayMemberProperty is not null || ValueMemberProperty is not null)
         {
+            // When the cell is in edit mode, check the item the user actually selected first.
+            // A linear search through the data source would return the first item whose display
+            // property matches, which is wrong when multiple items share the same display text.
+            if (OwnsEditingComboBox(RowIndex))
+            {
+                object? selectedItem = EditingComboBox.SelectedItem;
+                if (selectedItem is not null)
+                {
+                    PropertyDescriptor displayProp = DisplayMemberProperty ?? ValueMemberProperty!;
+                    object? selectedDisplayValue = displayProp.GetValue(selectedItem);
+                    if (selectedDisplayValue is not null && selectedDisplayValue.Equals(formattedValue))
+                    {
+                        value = GetItemValue(selectedItem);
+                        return true;
+                    }
+                }
+            }
+
             // Now look up the item in the DataGridViewComboBoxCell datasource - this can be horribly inefficient
             // and it uses reflection which makes it expensive - ripe for optimization
             item = ItemFromComboBoxDataSource((DisplayMemberProperty ?? ValueMemberProperty)!, formattedValue);