The Simic support says ‘Whenver a creature gets buffed, that creature gains “At the end of this turn, this creature gets +1/+1”, until end of turn.’
This support is triggering doubly for the opponent’s creatures - when an opponent’s creature gets buffed, it receives +1/+1 at the end of their turn and ALSO at the end of the player turn. This behavior does not happen for player creatures.
I think this the buff is happening for each time the opposing creature was buffed in one turn as well as the above. I’ve seen my opponent’s creatures get multi-buffed (+1/+1) when my creatures haven’t been buffed at all.