The Brain Stealers of Mars

This is a short story, probably from one of the other books written by John W. Campbell. It is about two men who are working with atomic power, who when outlawed flee to Mars with some exotic weapons they have developed. Once on Mars they discover a native life form the mimics any other life form, and also reads minds so it is impossible to separate one out of a group. With each of them having a number of identical duplicates, they determine to return to Earth, but first must eliminate the impostors. This is based on how they accomplished the task.