Add test for duplicate item persistence

tests/test_inventory_persistence.py

@@ -139,6 +139,24 @@ def test_load_player_data_empty_inventory(db):
     assert data["inventory"] == []
 
 
+def test_save_load_duplicate_items(db, test_zone):
+    """Duplicate items (two rocks) round-trip correctly."""
+    create_account("Dupes", "pass123")
+    player = Player(
+        name="Dupes",
+        x=5,
+        y=5,
+        location=test_zone,
+    )
+    Thing(name="rock", location=player)
+    Thing(name="rock", location=player)
+
+    save_player(player)
+    data = load_player_data("Dupes")
+    assert data is not None
+    assert data["inventory"] == ["rock", "rock"]
+
+
 def test_load_player_data_migration_no_column(db):
     """Old DB without inventory column returns empty list."""