This plugin finds places on images which have been altered with the clone tool.
It's based on the stand-alone copymove sourcecode from John Graham-Cumming:
This detector uses the technique described in:
'Detection of Copy-Move Forgery in Digital Images', Fridrich, Soukal and Lukas
Build and install it with:
$ CC=g++ CFLAGS=-O3 gimptool-2.0 --install elsamuko-copy-move.c
You'll find it then under Image->Copy Move.
Warning: The algorithm needs a lot of time to scan the image for duplicates, try it on small images first.
Also the image should be smaller than 1.5 MP, else you could run out of memory (and the plugin crashes).
For a Windows version and more example images go here:
Update1: Linux version with multithreading and can be used for bigger images now.
Update2: Massive speed gain thanks to the hint of Luca Del Tongo:
The calculation only uses the lower frequencies of the DCT now.
If you like it, hug someone.
Example image (cc) from Mark Hogan: