Update ImapDb.Account hack to remove duplicate inboxes
Two of my account's databases had duplicate inboxes which were not being removed by the existing workaroud, this updates it so that they are.
I had removed the duplicate from a third account ages ago and it hasn't come back since, so maybe the underlying problem is gone, but the correct fix would be to move this workaround to a migration and add a uniquness constraint on (parent_id, name).