Two different cusomers with the same tax/vat number

I have a store with magento 2.2.4 installed.
If a customer wants to register on the site, magento do a check on the e-mail and, if another customer was already registered with that e-mail, stops the registration process and shows a warning.
But i find that if a customer enter a Tax/Vat Number that is already registered, Magento doesen’t do the check, at least on my store with the current settings.
There is a way to force Magento to do this check and to not enable to enter a customer registration if the tax/vat number is already on the database?
This is a strange behaviour since you can have a situation where a customer can register 2+ times with different name/e-mail but the same tax/vat number.
There is a solution?
Thank you.