How to prevent importing duplicates FCP X

I can't seem to find an option to stop the importing of files that I've already imported. I've been on vacation for 1.5 weeks and have taken a variety of video with my SLR and iPhone. I put them all into folders based on location: El Paso, Ruidoso, Albuquerque, etc.

I'm home now. I know I have some video files on my iPhone I've not imported yet, but most I have. How do I know what has been imported and prevent importing duplicate video files?

I started to do a manual comparison, but the task is a bit daunting. Any secrets?

Thanks