I have a bunch of photos in LR 5 on a Mac broken out into folders by years.
My wife has a bunch of photos on an external drive many of which are duplicates of ones I already have (but with a different file name). Is there a way I can compare ALL of my photos to hers and then only import the ones that are not duplicates?
I've been using the excellent Mac app called PhotoSweeper http://overmacs.com/photosweeper.html
I had more than 40,000 photos, and thousands of them were either duplicates or similar. PhotoSweeper helped me to correctly identify thousands of images, which I then deleted.
There is also a LR plugin called Lightroom Duplicate Finder http://www.lightroom-plugins.com/DupesIndex.php
I have used only the trial version. It looks excellent.
However, I believe that it scans photos only in your LR catalog (either the entire catalog or any selection you make).
Therefore, you would need to import your wife's photos into LR (perhaps import them into a separate folder), run LR Duplicate Finder, then delete the duplicates.
Hope this helps.