Only Creature Phenomenon
The Only Creature Phenomenon is a concept that is somewhat ripped off from Transformers, much like a great deal of BZPB.
The One Creature Phenomenon Theory dictates that certain beings cannot have alternate versions of themselves in other realities. In other words, there is only one of that being in the whole Multiverse, as opposed to multiple versions of him/her.
More explanation may be highly desirable, but as this involves complicated science fiction that may end up causing head explosion, headaches, brain disruptions, plotholes and utter insanity, it is best to keep this as brief as possible.
It is based on the multiversal singularity concept from Transformers.