I'm not a great fan of the practice of routinely deleting duplicate files from PCs. Sure it can free up some disk space but it can also get inexperienced users into a great deal of trouble.
There are quite a few duplicate file detection utilities around but CloneSpy is the one that may impresses you. It's certainly not the fastest program in its class but it's definitely the smartest. First, you can specifically select what is to be scanned and this can include multiple drives, multiple folders or just individual folders. Second, it has the ability to detect duplicates by CRC, by file name, by CRC and file name and by file name and size. Third, it will also detect zero length files. Fourth, the program does not need to be installed but can be run from the executable so it's a good candidate for your USB Flash drive toolkit. Finally, it throws up duplicates to the user in a way that at least makes you think about what you are deleting. Click-happy users may find this latter feature an annoyance; to me it's a safety feature.
Safety feature or not, please read the help file and use with care. In particular do NOT include your Windows folder in your scan unless you are a knowledgeable technical user.
[1] CloneSpy
Website: http://www.clonespy.com/
Download link: http://www.clonespy.com/?Download
Author: CloneSpy
Current version: 2.3
Version date: October 21, 2006
License: Freeware
Download File size: 643KB
Operating Systems Supported: Windows 98se/NT/ME/2000/XP
Additional Software Required: None
64 Bit Capable: No
Non-English languages supported: None
Other relevant information: None
doublekiller is another contender worthy of mention
http://www.bigbangenterprises.de/en/doublekiller/