Can I delete incremental backups made with Déjà Dup?

Files associated with full and incremental backups:

A full backup contains all the data of that backup, and it does not depend on any other backups. It consists of at least three files:

• duplicity-full.DATE.manifest (just one)
• duplicity-full.DATE.volYYY.difftar (one or more)
• duplicity-full-signatures.DATE.sigtar (just one)

If the backup is encrypted, these filenames will have a .gpg suffix.

An incremental backup only contains the differences between a previous backup and itself, it depends on the previous backup, and of its previous backups, all the way until the chain reaches a full backup. It consists of three files:

• duplicity-inc.DATE.to.DATE.manifest (just one)
• duplicity-inc.DATE.to.DATE.volX.difftar (one or more)
• duplicity-new-signatures.DATE.to.DATE.sigtar (just one)

To delete a backup, you can just delete the set of corresponding files. Remember that all subsequent dependent incremental backups will become invalid. You can do this manually, or you can use the duplicity command.

Command overview of duplicity:

Here's how to view a set of backups using the duplicity command:

$ duplicity collection-status file:///path/to/backup
# output truncated for brevity

Found 0 secondary backup chains.

Found primary backup chain with matching signature chain:
-------------------------
Chain start time: Thu Nov 22 12:40:53 2012
Chain end time: Thu Jan 10 14:17:35 2013
Number of contained backup sets: 7
Total number of contained volumes: 358
 Type of backup set:                            Time:   Number of volumes:
                Full         Thu Nov 22 12:40:53 2012               162
         Incremental         Thu Nov 29 15:12:49 2012                 4
         Incremental         Thu Dec 13 09:05:17 2012                10
         Incremental         Thu Dec 20 11:09:12 2012                 9
         Incremental         Thu Dec 27 00:05:55 2012                83
         Incremental         Thu Jan  3 11:07:31 2013                79
         Incremental         Thu Jan 10 14:17:35 2013                11
-------------------------
No orphaned or incomplete backup sets found.

Here's how to remove backups older than one month, keeping any backups that are required for incremental backups in the last month:

duplicity remove-older-than 1M file:///path/to/backup

Here's how to remove all backups except the last full backup set, and its incremental backups:

duplicity remove-all-but-n-full 1 file:///path/to/backup

Here's how to remove all incremental backups except the last full backup set, and its incremental backups:

duplicity remove-all-inc-of-but-n-full 1 file:///path/to/backup

The manpage for the duplicity command is very informative, take a look.