Rule 603.2d

Some trigger events use the word "becomes" (for example, "becomes tapped" 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. Similarly, they don't trigger if an object enters a zone 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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