Unison

Unison is a backup utility that allows you to sync two directories.

Get it

Download it from http://www.cis.upenn.edu/~bcpierce/unison/

TroubleShooting

Inconsistent state

Symptom

Warning: inconsistent state.  
The archive file is missing on some hosts.
For safety, the remaining copies should be deleted.
  Archive ar12953235d4d7106b0880b820546c3ea1 on host tweedleburg is MISSING
  Archive ar6a548e7688b901a571b72c38e13194ff on host scorpio should be DELETED
Please delete archive files as appropriate and try again.

Solution

Go to ~/.unison and delete the archive file