If you have two Persons that exists twice in the system, you might get notified by the system duplicate validation. As default, the validation is setup to check e-mails on the Persons, and therefore it will not be possible to have two Persons with the same e-mail in the system.

You can change the criteria for the duplicate validation, by going to Configurations → Custom fields → Persons and click on the duplicate icon on the top of the page.

Be aware that the e-mail has to be the exact same, for the duplicate validation to work. If you should have the need for two of the same Persons in the system, you can change the datafields for the duplicate validation to Email and Contact-ID.