i suspect that you probably don't want that behavior since you try to remove all duplicates. I suspect you only check for duplicates when you first run the import script.
Import doesn't remove duplicates. All the videos of a movie should be listed inside the "owned videos" box. For example, you can have a movie as DVD, as BLU-Ray, as MKV file, etc... Have you checked the "owned videos" box? It's on the movie page, below the movie poster.
If there's an issue, it has to be with your import file. Are you importing from another app? Which one?
I know you don't allow concerts, but since these are full discs and are listed in IMDB, I wonder if they're allowed or not.
How many titles? I can be a little compliant, but I wouldn't want to be inundated with concerts...