Cross-login / sign up of users

Hello there!

I am experiencing a very confusing situation on a live app.

I’m using the login-link routine + the Fingerprint plugin from @vnihoul77 to log users in and keep them that way. So it happens that some users seem to be cross-signed up.

For example:
user Maria Hill with e-mail nickfury@avengers.com and
user Nick Fury with e-mail mariahill@avengers.com

Maria Hill logged in, made a purchase, paid with her credit card and never got her receipt. When I was checking the logs and orders I figured this out: the e-mail address was wrong.
But I did check with Maria Hill the order information and credit card last digits, so she’s clearly the owner of the order.

Has anyone ever seen such a thing? Or at least have any idea on where to start?