I have also noticed, that there are some duplicate folders which are only local. This should not be and is in my opinion a bug. It seems that the Mailsetup creates those folders maby because the online folder is not discovered correct via IMAP.
For me, it is:
“Gesendet” (Sent) is there twice with the same nametag
“Postausgang” (Outbox) twice with the same nametag
“Entwurf” and “Entwürfe” (Drafts vs. Drafts)
“Junk” and “Unerwünscht”
On of them is always only local and seems to be prefered target. It would be better, if one could assign folders manually like in Thunderbird.
EDIT: Seems to be a very old issue. Moving e-mails in IMAP account only done locally [closed] - together.jolla.com