This program is capable of finding duplicated files and turning one of them into a hard link to the other, effectively reducing the space usage (as the data will not be duplicated in the disk). It works only on POSIXish systems, and was tested in GNU/Linux only.
WARNING: As the duplicated files will point to each other, if one of them is modified then the other will as well! You should use make-hard-links only if the files won't be modified, only renamed, removed, or created.
The author had a Maildir with lots of duplicated files, and in his case make-hard-links was capable of reducing the space usage in 29% (as measured by "du -hcs" and "du -hcs -l"). As a final note, this program currently has a high memory usage as it has to maintain information about all the files that are possibly duplicates in memory at once (the author measured 1.7 GiB of memory for about 300.000 files).