http://duplicity.nongnu.org/duplicity.1.htmlyou can do all of those things.
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remove-all-but-n-full count
Delete all backups sets that are older than the count:th last full backup (in other words, keep the last count full backups and associated incremental sets). count must be larger than zero. A value of 1 means that only the single most recent backup chain will be kept. Note that --force will be needed to delete the files rather than just list them.
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full
Perform a full backup. A new backup chain is started even if signatures are available for an incremental backup.