Problem entering multiple customer VAT numbers

We have a UK Operating Unit subscribed to a UK tax regime.

We have a Dutch customer with numerous ship-to addresses.

The customer has a number of European VAT registrations (eg UK, NL, DE etc), and the customer VAT number we quote on on the transaction/invoice etc varies according to the ship-to address of the transaction. Each VAT number may be used across several of the ship-to addresses.

We are struggling to record the VAT numbers against the customer ship-to address.

In the Tax Profile on the ship-to site, in the Tax Registrations block, if we attempt to populate the Default Reporting Registration Number field (with the correct related country in the country field), we get errors re duplicate VAT numbers already existing.