Outlook is normally used in conjunction with an Exchange Server, eg work-based emails. If that is what your Outlook connects to, have a word with your email Tech person at work.
Otherwise, look through Outlook's Tools > Options, (or Tools > Account options), for some option that says, "Leave messages on the server" or similar. See if changing that solves the problem. If all email messages are deleted from the server once they are downloaded to Outlook, then they shouldn't be sent again.
I started getting two of each message received. What caused this and how do I fix it?