eatmymail
Finds and deletes duplicates of mails in maildirs.
Duplicate detection is based on Message-IDs and hashsums of content parts of each mail.
Beware: This is very much untested and very likely to eat your mails! Before you consider using this, make sure to have a backup and be prepared to restore it!
Also, don't blame me if it swallows your inbox :-]