I have multiple contacts with the same email address.
We recommend ensuring that there will be no differences in custom unique identifier fields when uploading large batches of contacts to Mautic.
In Mautic, email addresses are by default, a unique identifier for all contacts, meaning that in most cases, it will be impossible to have multiple contacts with duplicate email addresses. The only exception is if you have set up a new custom field and have this new field also set as a unique identifier. In this case, a new contact uploaded, that has an email that matches a contact already in the system, with a non-matching custom field will generate a new contact, rather than merging with the existing contact.