CiviMail should track unique opens

Not sure if this is a duplicate... I didn't see it in the issue list.

The report for a mailing tracks the opens. In practice, there are often multiple opens per user. The system tracks 5 opens from David Geilhufe as 5 opens. So I might send to 20 people and get 40 opens.

This is misleading since the primary reason for tracking opens is to see how many people are actually opening your mail.

Proposed solution: Keep the details list the same (showing the duplicates-- 5 opens by David), but make the total number of opens only unique opens (i.e. opens by unique email address).