The Quod Perierat Theory

Latin meaning: those lost to another dimension;
The complex and complicated theory that the same person from different dimensions is trapped in this one. These people all represent one person from alternate dimensions, but somehow, for unknown reasons, ended up in the same dimension. These people are very similar and only differ in the characteristics they received from their individual dimensions. For example, the same person could be the opposite sex in another dimension, but these people would still have similar personality traits and think in similar ways.

The original person who actually belongs in this dimension is aware of all of these "fakes", but is powerless and cannot get rid of them because if one of the fake copies dies, all of the others do too, as well as everyone else in the entire world. The universe's many dimensions rip apart because it realizes that these fake copies have come to coexist with the original. Therefore, the original isn't a real threat to the fake copies.
The fakes can kill the original. If the original is killed, one of the fake copies replaces them. The universe tries to fill in the blank of the original by using one of the fakes, and doesn't take note that the fake or actual original ever existed beforehand. The fake copy becomes the new original, but it's as if nothing occurred in the first place and as if the original never really died.

Todd: Hey, do you believe in the... *whispers* The Quod Perierat Theory?
Amanda: What? You know about it?!
Todd: Rick and I are pretty sure that Alice is our original.
Amanda: Oh my god. What are you guys going to do?
Todd: Actually, since we're the same person, it's more like just 'you'. I think I'm going to get rid of Alice.
Amanda: What does Rick want to do?
Todd: I mean, naturally, he agrees with me, just like on most things. Same person and all.