When you rehash something, you use it again without altering or changing it much. Your favorite band's new album will be a disappointment if they just rehash all their old hits.
Sometimes movie sequels are great films in their own right, but often they simply rehash the storyline, characters, and jokes from the original. You can call a movie like this a rehash. Another way to rehash is to go over something again, rethinking or reconsidering it: "She just wants to rehash that argument, trying to figure out why her friend got so angry."