Review for Remove Duplicate Messages by demaniak
Rated 3 out of 5 stars
I have mail that was migrated from GSuite into privately hosted IMAP server. Due to how mail is structured in gmail (labels etc) there is a fair amount of duplication. This addon did remove a lot, but a lot of seemingly obvious duplicates are completely ignored. These duplicates tend to have large attachments (10+Mb), if that is perhaps a factor. Otherwise they have identical subject lines, timestamps etc.
But credit due - the plugin DID help me eliminate maybe 10Gb worth of duplicates in that account, so it does kinda work.
To create your own collections, you must have a Mozilla Add-ons account.