Thriving Grove doesn't have any triggered ablities, so Yarok has nothing to make trigger twice. What the Grove has is a static ability that creates a replacement effect. A triggered ability always uses the words "when", "whenever" or "at". If neither of those words is present, you are not dealing with a triggered ability.
113.3c Triggered abilities have a trigger condition and an effect. They are written as “[Trigger condition], [effect],” and include (and usually begin with) the word “when,” “whenever,” or “at.” Whenever the trigger event occurs, the ability is put on the stack the next time a player would receive priority and stays there until it’s countered, it resolves, or it otherwise leaves the stack. See rule 603, “Handling Triggered Abilities.”
614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . ” are replacement effects.
Former Rules Advisor
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