Theoretical Duplication 
Instant
Whenever a nontoken creature an opponent controls enters this turn, create a token that's a copy of that creature.
Legality
standardNOT LEGALpioneerNOT LEGALmodernNOT LEGALlegacyLEGALvintageLEGALcommanderLEGAL
Instant
Whenever a nontoken creature an opponent controls enters this turn, create a token that's a copy of that creature.