dupmail scans a directory that (presumably) contains offline e-mail files which are usually fetched by programs such as offlineimap, mbsync or fdm. 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.
On exit dupmail will print the message id and file paths of all the duplicates.
# go build -o /usr/local/bin/dupmail main.go
dupmail [-d maildir] [-n]
-dmaildir to be scanned-nscan the maildir but do not delete any files