I remember very little from this movie. The threat, I think, was a kind of creature capable of taking over/replacing people.
At the end, the hero monologues that he wouldn't be able to trust anybody ever again because anyone could be a dupe, and that he would have to live on the run from now on.
It is probably from the 80s or very early 90s.
Not The Thing, The Hidden or any of the Body Snatchers