There was a SF short story in Playboy years ago that I never was able to track down again when I went looking for it. Had to do with a rocket full of astronauts landing on another air-based planet. That planet was inhabited by large worm-like creatures that used the self-defense mechanism of shape-shifting to look like the invaders, so there were hundreds of individuals on the planet surface who looked like the people in the crew. And the difficulties of telling who was the real crew member so they could try to leave. I remember there was something repetetive in it to do with "shoes."

SRS