We’ve made some updates to the Options –> Migrate IMAP feature that are currently on the beta server. Certain edge cases that were causing problems have been fixed up, and there’s now a “No duplicates” option as well. When enabled, as each folder is migrated, the migration code will first check if the folder exists locally, and if so, retrieve a list of Message-Id headers. It will then not download any remote message with the same Message-Id. This can be useful for avoiding large numbers of duplicate emails being downloaded if for some reason a migrate only partially completes, or for some reason you already have some messages downloaded from a remote server.

Please email me (robm@fastmail.fm) with feedback if you’re able to test this feature. Assuming there’s no issues, this will be rolled out to production soon.