Hotmail Duplicate Messages in sent folder

I use thunderbird (windows 10) as a client for 2 gmail and 2 hotmail accounts the problem I have is that all emails sent from the 2 hotmail accounts using thunderbird are duplicated (also on server outlook.com) to their respective sent folders and are identical apart from size which for the smaller emails is a difference of exactly 1.3kb so unable to delete with any duplicate email utility, when sent from hotmail/outlook.com on the web they are not duplicated, I guess it must have something to do with hotmail settings but not sure which, have experimented in the past without luck, am also concerned about manually deleting the duplicates as some are important without understanding what I am doing so any help would be appreciated.