External tool to find duplicated extens available https://github.com/markfasheh/duperemove
Simply do 'make' and 'make install' Following binaries will be installed:
Recursively find duplicates in <dir>
duperemove -r <dir>
The same as above, but make real deduplication (on BTRFS only)
duperemove -rd <dir>
Increase deduplication chance by reducing block size to 4k:
duperemove -b 4k -rd <dir>
This deduplication works nice, it can find some shared parts inside Maildir.