I have an iphone 4 and only one mail account:yahoo. When I receive Email messages, the number shown on the Mail icon in the dock is always double the number of messages I received. In other words,if I receive one message, the number 2 appears on the icon. If I receive two messages, the number 4 appears, etc. The same thing happens on the "Mailboxes" screen. What is causing this, and how can I correct it 