test: add test case for DN duplicate serial nos

erpnext/stock/doctype/delivery_note/test_delivery_note.py

@@ -1211,6 +1211,38 @@ class TestDeliveryNote(FrappeTestCase):
 
 		self.assertTrue(return_dn.docstatus == 1)
 
+	def test_duplicate_serial_no_in_delivery_note(self):
+		# Step - 1: Create Serial Item
+		serial_item = make_item(
+			properties={
+				"is_stock_item": 1,
+				"has_serial_no": 1,
+				"serial_no_series": frappe.generate_hash("", 10) + ".###",
+			}
+		).name
+
+		# Step - 2: Inward Stock
+		se = make_stock_entry(item_code=serial_item, target="_Test Warehouse - _TC", qty=4)
+
+		# Step - 3: Create Delivery Note with Duplicare Serial Nos
+		serial_nos = se.items[0].serial_no.split("\n")
+		dn = create_delivery_note(
+			item_code=serial_item,
+			warehouse="_Test Warehouse - _TC",
+			qty=2,
+			do_not_save=True,
+		)
+		dn.items[0].serial_no = "\n".join(serial_nos[:2])
+		dn.append("items", dn.items[0].as_dict())
+
+		# Test - 1: ValidationError should be raised
+		self.assertRaises(frappe.ValidationError, dn.save)
+
+		# Step - 4: Submit Delivery Note with unique Serial Nos
+		dn.items[1].serial_no = "\n".join(serial_nos[2:])
+		dn.save()
+		dn.submit()
+
 	def tearDown(self):
 		frappe.db.rollback()
 		frappe.db.set_single_value("Selling Settings", "dont_reserve_sales_order_qty_on_sales_return", 0)