When importing a WinBooks zip, contacts sharing the same number might fail VAT validation if distinct contact data is merged incorrectly.

Steps to reproduce:
- Create a database with the Belgian localization installed
- Install the account_winbooks_import module
- Navigate to Accounting > Configuration > Settings > Initial Setup > Import
- Import the WinBooks zip

Issue:

The import fails with the following error:
The VAT number [AAAAAA] for partner [BBBBBB] does not seem to be valid. Note: the expected format is BExxxxxxx.

Analysis:
In WinBooks, contacts can be of type Supplier or Customer, and it is possible for a supplier and a customer to share the same number. During the import, the system will merge contacts if found with the same number.

In the specific case, the VAT number from one contact without country is combined with the country information from another contact, resulting in a VAT validation error.
This change prevents the merger of partner data if their VAT numbers are explicitly different.

opw-6251772

Forward-Port-Of: odoo/enterprise#119579