How about artists I've seen exactly twice:
Emperor
Enslaved
WitTR
Boris
Cannibal Corpse
Wayfarer
Fleshgod Apocalypse
Municipal Waste
Warbringer
Allegaeon
Puscifer
Slayer
AFI
Bad Religion
Ozzy Osbourne
Cradle of Filth
Incubus
Slipknot
Macabre
BTBAM
Bongzilla
@anageorge.artfive artists you've seen at least twice:
Lady Gaga
Ghost
Murder by Death
The Birthday Massacre
Carpenter Brut