Hi YOG,
There are companies having offices on multiple locations. In some scenarios these separate offices will be entered as different customers to be able to register e.g. a primary address, contact person and details per office. In that scenario, they will have the same VAT number.
In my country, The Netherlands, we have the concept of fiscal units. Then companies owned by the same holding can apply for one shared VAT number. For sure, in this scenario you will need to be able to maintain customers with the same VAT number.
In some implementations, they setup the VAT number to be mandatory filled. In the case there is a company or person without VAT number, they will use e.g. 'None' as VAT number.
For these reasons, out of the box, there is no support for your requirement. You can achieve it with a customization, however, I would recommend to give a warning or a box where the user can choose to continue or stop and review their settings if there is an unattended duplicate or if it would be really required to have the same VAT number at various customers.
