There are a number of reasons as to why your Yahoo mail could be sending you the same emails repeatedly. First, it could be that the list of the downloaded message IDs got corrupted. Due to this, Outlook will lose track and start downloading all the messages afresh. It could also be that your email client is set "Leave Message on the Server". This will ensure that Outlook downloads all messages every other time. You can check solutions to these problems from http://www.cybermash.com/support/dox/receiving_email_messages_multiple_times.pdf.