Why do duplicate sub-folders appear in Thunderbird? In my personal folder I now have Sent and Sent Messages, bothe 'special' folders which can't be deleted. I have a similar situation in an folder for another email address (Sent + Sent Messages) AND in my Gmail folder, where not only are there 'Sent' & 'Sent Mail' folder but also two 'Drafts' folders as well as a 'Junk' folder and a 'Spam' folder. Each duplicate sub-folder contains slightly different messages and none can be deleted. See the attached image with the duplicates indicated.

What's going on and how can I resolve it to rationalize/simplify the structure?