fix: prevent TimestampMismatchError resolving Dunning with multiple overdue installments

`get_linked_dunnings_as_per_state` joins Dunning to its Overdue Payment child
table without DISTINCT. When a Sales Invoice has more than one overdue
installment, its Dunning holds one Overdue Payment row per installment, so the
query returns the same Dunning name once per row.

`update_linked_dunnings` then loads that Dunning name into a separate document
object for each duplicate row and saves each one. The first save bumps the
`modified` timestamp, so the second (now stale) save fails with
`TimestampMismatchError` ("Document has been modified after you have opened
it"). The error is raised on the Dunning while the user is submitting a Payment
Entry, making it confusing, and payments for such invoices cannot be posted at
all.

Add DISTINCT so each linked Dunning is returned (and saved) exactly once.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
(cherry picked from commit 06bfc23436)

erpnext/accounts/doctype/dunning/dunning.py

@@ -275,6 +275,7 @@ def get_linked_dunnings_as_per_state(sales_invoice, state):
 		.join(overdue_payment)
 		.on(overdue_payment.parent == dunning.name)
 		.select(dunning.name)
+		.distinct()
 		.where(
 			(dunning.status == state)
 			& (dunning.docstatus != 2)

erpnext/accounts/doctype/dunning/test_dunning.py

@@ -122,6 +122,41 @@ class TestDunning(ERPNextTestSuite):
 		self.assertEqual(sales_invoice.status, "Overdue")
 		self.assertEqual(dunning.status, "Unresolved")
 
+	def test_payment_against_invoice_with_multiple_overdue_installments_in_dunning(self):
+		"""
+		When an invoice has more than one overdue installment, its Dunning holds one
+		Overdue Payment row per installment. Submitting a Payment Entry for the invoice
+		must resolve the Dunning without raising a TimestampMismatchError caused by the
+		same Dunning being loaded and saved more than once.
+		"""
+		create_payment_terms_template_for_dunning()
+		# Post far enough in the past that BOTH installments (5 and 10 credit days) are overdue.
+		sales_invoice = create_sales_invoice_against_cost_center(
+			posting_date=add_days(today(), -15),
+			qty=1,
+			rate=100,
+			do_not_submit=True,
+		)
+		sales_invoice.payment_terms_template = "_Test 50-50 for Dunning"
+		sales_invoice.submit()
+
+		dunning = create_dunning_from_sales_invoice(sales_invoice.name)
+		# Two overdue installments -> two overdue payment rows for the same invoice.
+		self.assertEqual(len(dunning.overdue_payments), 2)
+		dunning.submit()
+		self.assertEqual(dunning.status, "Unresolved")
+
+		# Pay the invoice in full. This previously raised TimestampMismatchError on the Dunning.
+		pe = get_payment_entry("Sales Invoice", sales_invoice.name)
+		pe.reference_no, pe.reference_date = "3", nowdate()
+		pe.insert()
+		pe.submit()
+
+		sales_invoice.reload()
+		dunning.reload()
+		self.assertEqual(sales_invoice.outstanding_amount, 0)
+		self.assertEqual(dunning.status, "Resolved")
+
 	def test_dunning_resolution_from_credit_note(self):
 		"""
 		Test that dunning is resolved when a credit note is issued against the original invoice.