If you are thinking of an idea for a film then surely others are out there thinking and perhaps working on the same kind of idea. I suppose it makes sense because many of us are influenced by the same current events and societal issues. In other words, the same ideas are floating in the zeitgeist so it stands to reason that some of us would be thinking about, reacting to, and creating stories that feel familiar to each other.
It's still creepy when it happens -- especially when you haven't been aware of the other group's project until after each is pretty much done! And it's inevitable that the projects will be compared against one another and one will usually do better than the other. Though if both projects are strong perhaps the curiosity for comparing could help the two do even better than if they were released on their own. Who knows?
In any event, it's always intriguing when similar projects bubble up and I know it's stressful for the filmmakers as you don't want your project to be overshadowed and perhaps even stalled due to the competition. Many specs have gone unsold because someone else beat them to the punch by selling his or her script on the same topic or premise first.
It's an interesting phenomenon. One I'm sure that has intrigued anthropologists. Now all we need is the phenomenon that has us creating a box office smash with our low budget indie! I'm ready for that day!