Duplicates are easy enough - in Windows, I use a program called Duplicate File Finder (ver 1.1.0.0) by Rashid Hoda - it's a little hard to find these days, but it's a great program. Freeware as well.

Under Linux, I'd recommend just writing a script that does an MD5 hash check on files and then output the results. I wrote a VB6 program once to do a similar thing under Windows. The beautiful thing about open source is that someone has probably already written a script that will do MD5 hash checks under linux.

Also, for resizing of images, check out a ImageMagick - in particular, a program included called convert.exe (not to be confused with the Windows convert.exe program that converts the file system).

ImageMagick is great and will work under a variety of Operating Systems including *nix and Windows. It's quite powerful - and all command line as well, so it's perfect for scripts.