Me, myself, and I

A common way to unpack the notion that you have a counterpart in a parallel universe is that you and your counterpart had the same past up to a moment when a variable changes, resulting in you splitting into two of you with different life histories after that moment. 

In my admittedly limited experience with parallel universe fiction, you make contact with your counterpart, or the reader/audience observes your counterpart, some time after the split. A cliche example is that you're a hero in one universe but a villain in a parallel universe, yet we aren't shown what caused the difference. The story doesn't go back to the split. Parallel universe fiction frequently neglects the dramatic potential of retracing the two forking paths to the moment they split off. Two exceptions are The Butterfly Effect (2004) and Mr. Nobody (2009). 

In addition, there are two different ways to model forking paths. One is serial forking paths, where an individual goes through both doors at once, emerging as two copies in parallel worlds. Then that process continues at successive stages. Every so often he and his counterparts go through another set of doors up ahead. That generates exponential copies: 2, 4, 8, 16…

That would be difficult to write about or film because it quickly becomes unmanageably complex with too many diverging plots. 

Another is parallel forking paths, where it forks off at the same point in life. He comes to multiple sets of two doors at the same stage.

Say, there's the door where his parents divorce, and the door where they stay together. The door where the mother has custody and the door where the father has custody. The door where his brother commits suicide and the door where his brother doesn't commit suicide. The door where he marries his high school crush and the door where he misses out. The door where he wins the football scholarship and the door where he loses. The door where his blinded in a baseball accident and the door where he's not. The door where he becomes an atheist and the door where he becomes a Christian. In the latter scenario, that breaks the cycle. 

Although these are fictional alternatives in human imagination, they have a grounding in God's imagination. We never imagine anything God didn't imagine first.