There are infinite realities of everyone and everything. There can be a reality where Mario is Luigi even. If one person exists, for the most part there's infinite clones of that person. As Eyepatch Weegee discovered though, only 25 existed for Dark Mario. Dark Mario has an army of mainly Mario, Luigi, and Waluigi clones. Future Negative Mario threatened Dark Mario by saying he'd kill his past. But time travel isn't as simple as it seems. If Future Negative Mario killed Dark Mario's past, he'd just go into a reality where Dark Mario was of course killed.