Rule 603.2d

Some trigger events use the word “becomes” (for example, “becomes attached” or “becomes blocked”). These trigger only at the time the named event happens—they don’t trigger if that state already exists or retrigger if it persists. An ability that triggers when a permanent “becomes tapped” or “becomes untapped” doesn’t trigger if the permanent enters the battlefield in that state. Example: An ability that triggers when a permanent “becomes tapped” triggers only when the status of a permanent that’s already on the battlefield changes from untapped to tapped.

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