Outlook re-downloading server emails

hi there, 

About every 6 months my Outlook for mac starts downloading every single email I keep on the online yahoo server of the account I sync to Outlook instead of just the new ones. 

Does anyone know if this is an outlook issue or a yahoo issue and how to stop it happening? (at the moment the only way I can get my new emails is to let it re-download the 3000+ emails I have on my server)

thanks