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#11
I've used this free software for exactly the same situation and it works fine. Was able to find same images of different resolutions. You can change the filter for the search based on your preferences.
VisiPics
Faster than most commercial software
Uses dualcore processors
Highly efficient results with adjustable similarity levels
Easy to use Interface, preview your duplicates easily and pick the ones to delete with a simple click
Starts to display the results while scanning, you don't have to wait to delete your duplicates
Smart Auto-Select mode, to save time while deleting pictures
Tested on a 100.000 pictures 15Gb archive, full results in 3 hours only