I implemented the following:
1) If multiple preplaced blockers overlap, then just one of them becomes a blocker, while the others are walkers. Still I strongly encourage level designers to not create such levels in the first place. (Actually I coded this some time ago - not sure whether I already mentioned this somewhere)
2) A teleported blocker becomes a walker, if it would overlap an existing blocker.

I am still not absolutely happy with point 2), because it can be exploited in levels while normal players will never ever get to know about this game mechanics. Therefore if anyone makes such a level, I will personally visit them and bat them around with a large fishbot.