Well it may be true that all the plots have been written in one form or another. And you certainly find tropes reused in abundance between different series. This question asks, When did Star Trek reuse a plot from its own franchise; or a from another franchise or story? One that comes to mind is ENT: Doctor's Orders and VOY: One. Both episodes feature one crewman (Phlox and Seven) left alone to run the ship and hallucinate companions and antagonists. This plot was also used in Stargate SG1's Grace. Another is VOY's Rise, lifted from the movie(s) Flight of the Phoenix. What others come to mind?