Can riven give a buff to herself when she has a buff on her

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

Riven, Broken Blade's text says "Buff me. (Give me a +1 Might buff if I don't have one)," and units can only ever hold one buff (Rule 702.3.a). Choosing a unit that already has a buff is still a legal choice — it just doesn't place another counter (Rule 427.1.c). So the mode does nothing for her Might, and you've spent that option for the turn, meaning you'd normally pick Stun or ready instead.

Worth noting: because no buff is actually placed, abilities that read "when you buff me" won't fire off that redundant choice.