So by default Mautic unique identifier is email, this can however be changed to anything else you want.
We had numerous user cases where the client wanted to have multiple contacts with the same email address (for whatever reason, it is less relevant) and what we did was set a different field as the unique identifier, in our case we had something like unique_contact_id.
For us it was easy as the customer was generating their own unique_contact_id’s however a simple script in the middle can do this for you as well.
Hope this answers your needs