This is insane how Microsoft avoid question with usless non related detouring of debate. They instruct people anything just to drive them crazy or quit questioning any further out of desperation.
NO, U CAN'T DELETE DUPLICATES IN WLM.
But beauty of WLM is that stores emails in EML format. Use any file de-duplication software you want/use/know.
CAUTION:
WLM import email and rename them. So as many time you import them, it will import them making duplicates of duplicates. If Account is IMAP it will not accept deleted files from storage folder, but it will download them again.
- copy all emails on separate location or drive (twice if you feel affraid
- delete all email within WLM, so al folders will be empty (on IMAP that will reflect to delete them on server too, on POP3 ake shure all email are downloaded and backed up before deleting them)
- de-duplicate files on drive BEFORE importing them back into WLM
In folder in Windows you can sort EML files by subject, date, from, to, ... you just need to enable thoose columns (like normally you have filename, date modifyed, type, size... You can even make preset to one of five folder customization (general, music, documents, ...).
I hope this helps.
Thunderbird can't sync with new Outlook.com, eM client can, but email are in database... So I still use WLM under W7 in 2016/9. This tutorial is for myself or any other it might coma across later. WLM have its shorcommings but at the moment there is no greater substitute in all meanings. I still can sync contacts under NEW outlook account with my phone. Just for callendar I use eM client. For general email i use WLM, until someone writes proper APP.