Historical Rules · 17 June 2011
Rule 603.2d
Official source text from the 17 June 2011 Rules release
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.