Recently, I found several hundred email messages "recovered" in MAIL after doing a clean install.

Using a random selection of several, the originals were still in their original folders as well as appearing in the "new" Recovered Messages folder in MAIL.

Can all of these messages (duplicates?) safely be deleted from the Recovered Messages folders WITHOUT the originals being deleted from their original locations?

I have done clean installs many times before with each of the subsequent versions of OSX installations as far back as Tiger. However, these "recovered messages" have only appeared this time when installing HIGH SIERRA.

Any help would be appreciated. I'd hate to think I would have to check each one - of hundreds - on a one-at-a-time basis just to be certain.

those should be duplicates an be safe to delete, but I don't take rash actions with other users' data.

you can select a folder and then up top select Mailbox and Export Mailbox to back up your data if you want to.

or have time machine...

what's going on there is when mail updates, it scans all your folders for items, and cross-matches them with the index plist in each folder. if it finds a message that's not in an index, it moves it to the recovered folder. so what usually causes these is if mail crashes when you are moving a message from one folder to another, it temporarily exists in both places, and so if it crashes after copy but before delete, you have the dup. And since it's not in the source's index anymore, it won't show up unless you select the folder and rebuild it. Or update mail.

Super. Makes sense to me.