dupmail scans a directory that (presumably) contains offline e-mail files. The program deems a mail a duplicate by comparing the Message-ID header and in case there isn't one the mail is simply skipped.
These mails are typically fetched by a program such as OfflineIMAP, mbsync or fdm and should work nicely with any of them.
On exit dupmail will print the file paths of all the duplicates.
$ go build -o dupmail dupmail.go
# cp dupmail /usr/local/bin/
dupmail [-d maildir] [-v] [-n] [-s]
-dmaildir to be scanned-nscan the maildir but do not delete any files-sprint a summary instead of the file paths of the duplicates-vincrease verbosity. Dupmail will print out the duplicates in a very script friendly "<Message-ID> <TAB> <File Path>" format.