Updating entry creates a new entry

It’s quite simple: create a form, as the admin, then fill it in and save it. After that, create a wordpress user and assign the form to that user, via Edit form. Then login as that user from a different IP address and edit “your” form. It will save it as a new form instead of updating the old one.
I’ve disabled ajax in my form, I’ll get back with some feedback if this resolves the issue or not.