Problems with contact registration

I have an endpoint developed by me in my website, which allows me to send information from a form and this is sent to the endpoint and the endpoint sends it to constant contact.

The problem is when I want to register the same contact (email) in different lists, sometimes it lets me register it in 3 lists, or 2, or only 1 because the other lists give me the error “this email is already registered”.

This is a problem that occurs both in my endpoint and when I send the information directly through postman.

This is not a problem with my form, but is presented directly in constant contact.

Is there anything I can do?