There’s another way they can be similar. When I start reading and I step and check what I’m doing because I think I’ve read that one already, that’s a bad sign. What editors and agents are looking for is unique projects with unique voices. Now I know there are a limited number of base plots, but there are a myriad of ways to treat those plots, settings and writing. If I think I’ve already seen it chances are it is too similar.
That’s a shame when an author sits down and writes something they think is quite unique only to find out there are a ton of them coming in that are quite similar to one another, and we see that a lot. Maybe a movie or TV show hit a number of months ago, or an event in the news got a lot of people thinking along the same lines. Whatever the stimulus, all of a sudden a lot of proposals hit that are very similar to one another. It happens a lot, and when it does none of them may end up being chosen as editors and agents think “with this many coming in there are bound to already be some in the pipeline by now."
When that happens the author may end up having to put it in a drawer to pull out some time later when it has a chance of better standing alone. It’s all about the fit in the marketplace. Most, however, don’t think with a career view but keep trying to push it in and then possibly self-publishing it instead of trying for the big score again when the time is right.
Too similar can be a particularly frustrating response to receive to a submission.