Vanguard Seraph

Creature — Angel Warrior

Flying

Whenever you gain life for the first time each turn, surveil 1. (Look at the top card of your library. You may put it into your graveyard.)

Vanguard Seraph's last ability triggers just once for your first life-gaining event on each turn, no matter how much life you gain.
If you gain life during a turn before Vanguard Seraph is on the battlefield, its last ability won't trigger that turn even if you gain life again later in the turn.