riven broken blad can choose buff even if she already have?

Yes — she can choose the Buff mode, but it won't actually give her a second buff.

Riven, Broken Blade's ability says "Buff me. (Give me a +1 [M] buff if I don't have one)," and units can only ever hold one buff — choosing a unit that already has one is legal, it just isn't Buffed again (Rules 427.1.c, 702.3.a). So if she's already buffed, that choice does nothing, and since the mode now counts as "chosen this turn," you've spent that option for the turn — you'd normally pick Stun or ready instead.

One edge case worth noting: the same "nothing happens" logic applies to triggers that care about being buffed (Rule 427.1.c example) — so effects that read "when you buff me" won't fire off a redundant Buff choice.