Open source or free licence:
Proprietary or restricted licence:
This utilities only search for duplicate files:
$ sudo apt-get install fdupes
See:
Freedups searches through the directories you specify. When it finds two identical files, it hard links them together. Now the two or more files still exist in their respective directories, but only one copy of the data is stored on disk; both directory entries point to the same data blocks.
Dupmerge reads a list of files from standard input (eg., as produced by "find . -print") and looks for identical files. When it finds two or more identical files, all but one are unlinked to reclaim the disk space and recreated as hard links to the remaining copy.
ssdeep is a program for computing context triggered piecewise hashes (CTPH). Also called fuzzy hashes, CTPH can match inputs that have homologies. Such inputs have sequences of identical bytes in the same order, although bytes in between these sequences may be different in both content and length.