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.

```
ValueError: too many values to unpack (expected 1)
  File "odoo/models.py", line 5199, in ensure_one
    _id, = self._ids
ValueError: Expected singleton: account.move.line(516, 517)
  File "odoo/http.py", line 2116, in __call__
    response = request._serve_db()
  File "odoo/http.py", line 1698, in _serve_db
    return service_model.retrying(self._serve_ir_http, self.env)
  File "odoo/service/model.py", line 134, in retrying
    result = func()
  File "odoo/http.py", line 1725, in _serve_ir_http
    response = self.dispatcher.dispatch(rule.endpoint, args)
  File "odoo/http.py", line 1923, in dispatch
    result = self.request.registry['ir.http']._dispatch(endpoint)
  File "odoo/addons/base/models/ir_http.py", line 154, in _dispatch
    result = endpoint(**request.params)
  File "odoo/http.py", line 715, in route_wrapper
    result = endpoint(self, *args, **params_ok)
  File "addons/web/controllers/dataset.py", line 28, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "addons/web/controllers/dataset.py", line 24, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "odoo/api.py", line 461, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "odoo/api.py", line 448, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 923, in onchange
    return super(BankRecWidget, self.with_context(recursive_onchanges=False)).onchange(values, field_name, field_onchange)
  File "odoo/models.py", line 6529, in onchange
    record._onchange_eval(name, field_onchange[name], result)
  File "odoo/models.py", line 6240, in _onchange_eval
    method_res = method(self)
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 659, in _onchange_todo_command
    self._ensure_loaded_lines()
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 1481, in _ensure_loaded_lines
    self.line_ids
  File "odoo/fields.py", line 4287, in __get__
    return super().__get__(records, owner)
  File "odoo/fields.py", line 2793, in __get__
    return super().__get__(records, owner)
  File "odoo/fields.py", line 1155, in __get__
    self.recompute(record)
  File "odoo/fields.py", line 1365, in recompute
    apply_except_missing(self.compute_value, recs)
  File "odoo/fields.py", line 1338, in apply_except_missing
    func(records)
  File "odoo/fields.py", line 1387, in compute_value
    records._compute_field_value(self)
  File "odoo/models.py", line 4312, in _compute_field_value
    fields.determine(field.compute, self)
  File "odoo/fields.py", line 99, in determine
    return needle(*args)
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 231, in _compute_line_ids
    wizard._lines_widget_add_auto_balance_line()
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 1021, in _lines_widget_add_auto_balance_line
    auto_balance_line_vals = self._lines_widget_prepare_auto_balance_line()
  File "home/odoo/src/enterprise/saas-16.2/account_accountant/models/bank_rec_widget.py", line 972, in _lines_widget_prepare_auto_balance_line
    = self.st_line_id._get_accounting_amounts_and_currencies()
  File "addons/account/models/account_bank_statement_line.py", line 492, in _get_accounting_amounts_and_currencies
    transaction_amount = -suspense_line.amount_currency
  File "odoo/fields.py", line 1151, in __get__
    record.ensure_one()
  File "odoo/models.py", line 5202, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
```

Applying the changes will resolve the issue.

sentry-4217912848

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Forward-Port-Of: odoo/odoo#123405