There Can Be Only One — Ruling

Effetto 0

You cannot activate it during the Damage Step.
If a player has a face-up monster in their Monster Zone, they cannot Normal Summon, Flip Summon, or Special Summon face-up, a monster of the same type as that monster. (Normal Setting, and Special Summoning face-down, are still possible.)
If there are two or more face-up monsters of the same type in either player's Monster Zone, their controller must send monsters to the Graveyard so they only control one monster of that type. (This does not start a Chain. Monsters sent to the Graveyard as a result are not treated as being sent to the Graveyard by a card effect.)

About this card's ① st effect:
This effect is not an activated effect. It is applied by this face-up card in the Spell & Trap Zone.