Add "keep unique backups only" option

I had a thought that this would be a nice addition. Add an option so that after creating a backup, if any of the other backups have the same hash, remove the old ones with the same hash.

Pros:

• Saved disk space, without losing any backup data, since the unique backups are preserved
• The date of the backups would still show that backups are occurring if you keep the newer one

Cons:

• Potential implications for scheduled backups, if the number of backups isn't increasing during low site usage?