this is a common problem with outlook, which is not always easy to fix, Start by checking Rules to be sure that copies are not being made to the Inbox, and make sure that all your rules end in "Stop Processing" actions.
Then check the task manager to be sure that there is only one instance of Outlook running. Two instances of outlook.exe running can produce two messages arriving at the same time. You will need to then determine why two instances of Outlook were running in the first place.
Another thing you can do is to delete/remove ALL your e-mail accounts and recreate them. This worked for some people.
This is a link to a microsoft web page for this problem click here
OR try Firefox instead its just as good if not better Ive never have a duplicate.