Yep, that happened to me a couple of times: buying a CD or a DVD that I already have.  Maybe because I'm geeting older and my memory is not what it was.  Yikes!  Nah, I just have too many items, that must be the reason 

I started looking for a way to catalog all of this when I found the Collectorz.com Website.  They have software for CDs, DVDs, books and more.  I downloaded trial versions and liked the software especially how it retrieves information from Amzon and IMDB.  This means a lot less typing.  Still, you have to manually enter the movies names or barcode unless you use their barcode scanner so I decided to give it a try.  The scanner is very small and what's great, you can detach the cable, go scan your stuff where it is and come back to your computer later.  The barcodes are stored in the scanner's internal memory.  They say that the internal memory can store around 500 barcodes.

The only drawback is that the scanner comes with a serial cable and it's kinda hard to find serial ports in today's laptops.  The Collectorz Website state that you can connect the scanner to a USB port with the included cables but this is wrong.  I had to buy a serial to USB cable.  So be prepared to spend an additional $20.  I bought a Trendnet TU-S9 converter and I worked without problems.

[UPDATE: I did received the cable: http://guy.dotnet-expertise.com/PermaLink,guid,722ac58e-fbde-41cb-b5c2-c281e4ddeeaf.aspx]

Once you're done cataloging, you can export the list to HTML and display your collection on a Website or your mobile device.

If you don't want to spend a dime cataloging your movies, you can use Ant Movie Catalog, a free open source software.