fdupes and fslint are good tools. Both use md5 hash or/and size to compare files and then identify duplicates. fdupes is commendline too, fslint has nice user GUI. My personal preference is fdupes. As I can use the out for further process.
Install
sudo apt-get install fdupes # for ubuntu
or
sudo yum install fdupes #for fedora
use of fdupes, there are many options, use fdupes –help to find detail. my use is simple
fdupes ~/DIR -r -S > dupes.txt
