Of course it is a duplicate. The software doesn't catch all of those; that's on purpose, to allow good stories multiple cracks at the bat (edit: i.e. multiple chances to get attention). We rely on users to search before posting and on moderators to bury the dupes that sneak through. User comments pointing out the duplication are helpful.

This isn't a perfect system but it's much better than being too strict about reposts. We're going to work on a more effective dupe detection system as soon as we can get to it.