There are some pretty famous cases of movies redoing the plot of an earlier movie, or copying characters and details. But if your movie takes just a detail here and an idea there, we often won't notice. That goes double if your movie isn't a big hit. If you don't see a lot of movies, you'll miss out on some of these "twin" plots, and if you see every movie, you might not be able to keep up with them all. For example, I never saw Up but I saw Gran Torino. I know, that's opposite most people's experience. But someone, somewhere, will notice how two movies are similar in one way or another.
Some of these are surely coincidences, while others are simply re-using an idea that worked out before, and some are blatant rip-offs. Check the list of 34 instances at Cracked, because you certainly haven't noticed all of these.