Log into your mailbox using a web interface. Delete all of the unneeded messages and create another server-side folder. Move to that folder all the remaining Inbox messages that are not new. Because they're not in the Inbox, Outlook won't be able to see them and won't try to download them again. They're still be on the server and you'll be able to access via the web interface.
This happens if you create a new mail profile. Outlook loses the information that tells it what messages have been downloaded before.