I'm trying to find a story that I read around 1970 (my best guess would be 1968-1971) in Read magazine, which was a little magazine that we used to get in junior high school.
The details are hazy at this late date, but it was something about a guy who awakens one night to find that his body is being "eaten" in some way, starting from his toes. Little by little his body disappears or something -- I don't remember the fine points of it.
While this happens, he's talking to his evil doppelganger, who stands by his bed. The doppelganger is replacing him, and tells the guy in bed that he has to "eat" his body before he can replace him. Something like that. I think it ends with the guy wanting to scream, but being unable to because he has no mouth left.
It was terrifying to this 11-year-old!
Anyone remember this? No, it's not the similar Bradbury story Fever Dream.