1972 Ike Dollar
Flip Over Double Struck in Collar
PCGS AU 58
Although there are many different types of double struck coins, those that are flipped over are scarce. This Ike Dollar which was double struck in the collar (which flipped over between strikes) is very dramatic since Eisenhower's portrait is clearly visible underneath the eagle.
It is not unique, but is one of the more visual and eye appealing double struck Ike Dollars known.