Improve Vat No Validation

I would suggest to apply it to CR, the idea here in KSA, it is possible to have ONE VAT number and multiple CRs. Because CR = to Business activity. where VAT number is only one for each owner.
lets say if one person has more than one business, he will get ONE VAT number and multiple CRs.
If I am dealing with this owner and his businesses and trying to create them in ERP, I will need to have the VAT number duplicated but not CR

We have added 15 character length validation for VAT/Tax Number field in customer and vendor details form. It can be left empty or should be filled in with 15 characters. Duplicate VAT/TAX Numbers are not allowed.