New Dupes without selecting them

In my current world I have this strange behavior where suddenly there is one more dupe than I actually picked. 
The first time it happened I thought I just forgot that I picked a dupe (I was wondering why there were not enough beds and I haven't picked a job for that dupe).
Now the second time happend and I really don't know how.
I unfortunately don't know when and how this happend or if I accidentally pressed the key combination to spawn a dupe, but I don't think so.