I found a freeware program called DupKiller that I used to free up 10GB in a couple of hours.
Setting up the search parameters to find duplicate file sizes rather than file names produced much better results. I also set the parameters to sort the results by file size so that I could deal with the largest duplicates first to free up more disk space faster.
As you delete files, the duplicates disappear from your list, and when you exit the program, your list is saved so you can resume where you left off without having to scan your drives again.
There may be a better application out there, but this one is working nicely for me, and you can't beat the price.