Recently Yahoo made some changes in how you access your Yahoo Plus mail through Outlook. The Yahoo Concierge Team sent the links below, and sure enough the one I followed (Access Yahoo Mail in Microsoft Outlook 2007) worked!
All of a sudden I had to log into Yahoo Mail everytime I tried to access it. Finally I realized that Yahoo Mail had made a change and the bookmark I had was no longer taking me to my inbox, but to the logon page instead. So, do this: Log-in to your yahoo mail, from the the inbox page set a new bookmark (I just drive the URL to my toolbar). Finally, back to normal!
If you have an alias (another email address) setup for your yahoo account, and leave your email on the server, you will get the email twice (once per each Alias). You'll need to configure the alias account so that it does not receive emails.
In Outlook 2007 and earlier:
Tools > Options > tab Mail Setup > Send /Receive button > Edit > select the alias account and uncheck Receive mail items.
In the new version of Yahoo Mail (not the 'classic' version):
1- Click the "Switch View" box
2- Select "In Scrolling List (Hide Preview)"
3- Click the little checkbox directly under "Delete"
4- Click "Delete"