[FIX] account: prevent account_move_line singleton error

When '_seek_for_lines' returns multiple suspense_line for
transaction_amount calculation, then the user will face this
issue 'Expected singleton: account.move.line(516, 517)'.

Steps to reproduce:
- Create a new transaction in Journal (Bank) by Accounting Journal
> Bank Reconcilation > Add a Transaction.
-  While creating a transaction add a partner, amount (amount > 0),
Foreign Currency, Amount in currency and then save it.
- Go to Journal Entries by Accounting > Journal Entries
and then open your  recently created journal entry.
- Click on reset to draft and then in 'Journal Items'
add a line with some credit amount such that total credit and debit should be
equal. (Under Account it should be Bank Suspense Account).
- Now click on 'Bank Transaction' button in the form view.

By following above steps you will be able to reproduce the error.

Applying the changes will resolve the issue.

sentry-4217912848

closes odoo/odoo#126284

X-original-commit: 12a75a8521f8150c382645215c440d39af4b50aa
Signed-off-by: Laurent Smet (las) <las@odoo.com>

addons/account/i18n/account.pot

@@ -12346,6 +12346,15 @@ msgid ""
 "In doing so, you might want to resequence your entries in order to maintain a continuous date-based sequence."
 msgstr ""
 
+#. module: account
+#. odoo-python
+#: code:addons/account/models/account_bank_statement_line.py:0
+#, python-format
+msgid ""
+"%s reached an invalid state regarding its related statement line.\n"
+"To be consistent, the journal entry must always have exactly one suspense line."
+msgstr ""
+
 #. module: account
 #. odoo-python
 #: code:addons/account/models/account_move.py:0

addons/account/models/account_bank_statement_line.py

@@ -680,8 +680,12 @@ class AccountBankStatementLine(models.Model):
                         'amount': liquidity_lines.balance,
                     })
 
-                if len(suspense_lines) == 1:
-
+                if len(suspense_lines) > 1:
+                    raise UserError(_(
+                        "%s reached an invalid state regarding its related statement line.\n"
+                        "To be consistent, the journal entry must always have exactly one suspense line.", st_line.move_id.display_name
+                    ))
+                elif len(suspense_lines) == 1:
                     if journal_currency and suspense_lines.currency_id == journal_currency:
 
                         # The suspense line is expressed in the journal's currency meaning the foreign currency

addons/account/tests/test_account_bank_statement.py

@@ -524,6 +524,20 @@ class TestAccountBankStatementLine(AccountTestInvoicingCommon):
             with self.subTest(params=params):
                 assertAppliedRate(*params)
 
+    def test_for_presence_single_suspense_line(self):
+        statement_line = self.env['account.bank.statement.line'].create({
+            'journal_id': self.bank_journal_3.id,
+            'date': '2019-01-01',
+            'payment_ref': 'line_1',
+            'amount': 0.0,
+        })
+
+        with self.assertRaises(UserError):
+            statement_line.line_ids = [Command.create({
+                'account_id': statement_line.journal_id.suspense_account_id.id,
+                'balance': 0.0,
+            })]
+
     def test_zero_amount_statement_line(self):
         ''' Ensure the statement line is directly marked as reconciled when having an amount of zero. '''
         self.company_data['company'].account_journal_suspense_account_id.reconcile = False