Rule 426.1.c

Units with Buff Counters can still be chosen for actions that Buff units, but will not be Buffed as part of the execution. Example: A spell reads "Buff a unit. Then, if it was buffed this way, draw a card." It will place a buff counter on a unit that has no buff counter, and then its controller will draw a card. However, if the player chooses a unit with a buff counter on it already, then the card will not be drawn. Example: A unit reads “When you buff me, ready me.” If that unit is chosen for a game effect that would buff it and it is already buffed, its ability will not trigger.