As of right now it's a tie between Megadeth and Rob Zombie both of whom I've seen 3 times. The only other bands I've seen more then once are Exodus and Children of Bodom whom I've seen twice unless you want to count Zakk Wylde since I've seen him with both Ozzy and BLS. Every other band I've seen has just be once but I will be seeing Maiden for the second time.
May 4th - Volbeat, Anthrax
September 20th - AC/DC