Avoid Expected singleton error in change_partner_emails.py
CHANGELOG:
### Fixed
- [#538](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/538) Avoid expected singleton error in `_search_or_create_email` with duplicated child emails
CHANGELOG:
### Fixed
- [#538](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/538) Avoid expected singleton error in `_search_or_create_email` with duplicated child emails