I'm trying to remember a classic-era short story about a Martian landing I read in an anthology sometime during the late 1980s-early 1990s. It was an old anthology, probably from the 60s. I don't think it's any of the Bradbury stories.
In the story a landing party from Earth arrives on Mars and encounters Martians that can alter their appearance to mimic anything. One of them may have been imitating a piece of the astronaut's equipment at some point. They end up with multiple copies of themselves trying to identify the real earthlings before vaporizing the alien impostors and returning to Earth.
I think there were 2 astronauts and I don't remember how the first one was identified, but I remember some dialogue about the second where the other guy says something like "What you did required the coordination of hundreds of muscles across most of your body, X number of reflexes, brain areas... I took the gamble that the alien's shapeshifting wouldn't be able to replicate this" Then the other guy asks "What did I do?" and the last line of the story is "You sneezed."