I use EagleFiler to archive Apple Mail (MacOS 10.14.6, EF 1.8.10). I do this by creating a smart mailbox that has 6 months of messages, F1-ing it into EagleFiler, merging it with an older like-named mailbox.
Apparently a side effect is a mailbox in the EF trash. Over time, for instance, starting with a mailbox called “Other”, I’ve accumulated “Other-2”, “Other-3” etc. The messages in these trashed mailboxes appear to be duplicates of those in the merge target. Maybe after merging A1 into A, EagleFiler moves A1 to the trash. Want to be sure before I empty the trash.