I use Mercurial for version controlling my source code. But some people prefer other version control systems (like git, Bazaar, SVN, CVS).
I would like to know, is it possible to store a repository under multiple systems at once, so people can use whichever repository they want?
GitHub developed the hg-git extension for Mercurial. This extension allows you to clone from git repositories using Mercurial, and then push back in. So if you didn't mind having a central git repository, then it would certainly be possible to access it using both git and mercurial.
Though as another answer has pointed out most of the VCS tools you mentioned are fairly similar to use, so it's questionable whether it would be worth putting in the effort to allow people to use whatever they want, compared to the minimal effort involved in learning a new system.