Thunderbird saves sent messages twice
so a few months ago Thunderbird started to save emails I send from my mailbox.org account twice. Before that everything was working as intended, and I don't recall ever changing any settings.
Google hinted me to disable the "Place a copy in:" option in the "Copies & folders" section, but then no sent messages are saved. This problem does not occur with other mail accounts and other email clients don't seem to have that problem (with the mailbox.org account) either.
This happens both on my Linux and Windows system, I have already tried to delete and reconfigure the account in Thunderbird.