you can punch one person/thing in the face, who do you punch?
personally, i would punch my punch Me from 15 minutes ago. 15 minutes ago, i would have been just sitting on the computer, when suddenly another me appears out of nowhere and suddenly punches me in the face, then disappearing, and i would just be going 'WTF?!'. then 15 minutes later i would discover that i can punch someone in the face, so being that i am still pissed that Me punched Myself in the face, i would go back 15 minutes to punch myself in the face to get revenge, thus creating an infinite loop of me punching myself that may very well shred the fabric of spacetime =)