Hi Thomas,
I've taken a look for you, and I notice that you have around 30 custom plug-ins. The issue is almost certainly caused by one of those. What I suspect is that a memberSync extension has broken code, or perhaps code that tries to load a non-existent member without correct error checking is creating new members.
I would disable all plug-ins, and then when you have verified the issue stops with the plug-ins disabled, re-add them one at a time until you find the culprit.