There’s no constraint preventing duplicate `ir.default` records. When setting a default using `self.env['ir.default'].set()`, it searches for an existing one, but if more than one match is found, accessing `default.json_value` raises a singleton error.

This fix makes sure the search only picks one record, avoiding that crash.

Before fix:
```py
self: res.users(1,)
>>> company = self.company_id
>>> company
res.company(1,)
>>> self.env['ir.default'].create({'field_id': 4540, 'company_id': company.id, 'json_value': 7})
ir.default(9,)
>>> self.env['ir.default'].set('res.partner', 'property_account_receivable_id', 7, company_id=company.id)
Traceback (most recent call last):
  File "/home/odoo/odoo/odoo/odoo/orm/models.py", line 5630, in ensure_one
    _id, = self._ids
    ^^^^
ValueError: too many values to unpack (expected 1)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/odoo/odoo/odoo/odoo/addons/base/models/ir_default.py", line 107, in set
    if default.json_value != json_value:
       ^^^^^^^^^^^^^^^^^^
  File "/home/odoo/odoo/odoo/odoo/orm/fields.py", line 1670, in __get__
    record.ensure_one()
  File "/home/odoo/odoo/odoo/odoo/orm/models.py", line 5633, in ensure_one
    raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: ir.default(4, 9)
```

After fix:
```py
self: res.users(1,)
>>> company = self.company_id
>>> company
res.company(1,)
>>> self.env['ir.default'].create({'field_id': 4540, 'company_id': company.id, 'json_value': 7})
ir.default(10,)
>>> self.env['ir.default'].set('res.partner', 'property_account_receivable_id', 7, company_id=company.id)
True
```

opw-5228419


---
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: odoo/odoo#234944
Forward-Port-Of: odoo/odoo#234893