On my new MacBook Air running Lion all of the addresses on Address Book doubled. All of my other Macs are iMac late 2009 and a MacBook Pro and they are still running Snow Leopard.
It's probably iCloud. On the left column choose either All iCloud or All On my Mac and see if it reverts to single entry.
Because Mountain Lion Stings. Stick to MacLeopard. Mountain Lion : the worst Apple System ever.
Retrieving data ...