Aperture - is there an easy way to prevent and delete all the duplicates

I'm currently using Aperture 2 on an iMac (I use Nikon Capture DX2 on my MacBook) and have recently encountered an annoying problem.

When uploading photos from a memory card, Aperture is duplicating each photo, so instead of having 800-1000 in an an album, I have 1600-2000 photos.

How can I stop this happening, and is there an easy way to delete all the duplicates? (At the moment the only solution I can find is to manually delete each individual duplicate which is time consuming)

Thanks in advance for your help.

This might not be aperture's fault, you shooting nikon i presume.
Do you have your camera setting on for RAW+JEPG?

RAW only

It didn't used to make duplicates, and as far as I'm aware I've not changed any settings. What I've noticed is that in spite of selecting the card to download from, Aperture shows it "downloading" from my printer/scanner for some reason.