I've integrated Facebook's AccountKit as the login system for my app. It has some bugs but usually works fine.
My developer reported a fairly bad experience though recently. He had to restart his computer, thus clearing the sessions, and when he went to re-login to the app via AccountKit, he realized that the same email address he had been using returned a different user_id than the one it had been returning previously. This of course caused a problem in that our backend could not find any account relating to the new Accountkit id.
I've been unable to find much information about this. I'm wondering if this is a bug or a feature! Any further illumination on this subject is greatly appreciated!
Every login will return a different Access Token or Authorization Code as documented here: https://developers.facebook.com/docs/accountkit/accesstokens
If you get the Account ID from the Access Token, it should always be the same for a user entering the same email address. What you're describing sounds like a bug and if you can still reproduce it, you should submit a bug report and someone will look into it. https://developers.facebook.com/bugs/