This may be the issue and the instructions in the added document may help.
Try reading the information and some of the info may help with your issue. KB04258-How to remove an integrated email address from the BlackBerry Internet Service
The BlackBerry smartphone is receiving duplicate email messages from one or more Yahoo!® Mail accounts.
BlackBerry® Internet Service
One of the Yahoo! Mail accounts is the original account and the others are aliases of the original. Since they are aliases, they share the same inbox on the Yahoo!® messaging server. This means that the same inbox is checked multiple times and the same email messages will delivered for each alias.
Remove all but one of the Yahoo! Mail accounts.