I figure that Gary would know the answer to this...
Yesterday I was listening to the radio, and they played a recording from Minnesota of Debussy's Prelude to the Afternoon of a Fawn.
You can hear an excerpt of the piece here, though it's not the same performance that I had heard, and you can't hear the double gliss clearly.
The harpist played some beautiful glissandos with each note struck twice. I assume that this was done with one hand strumming right behind the other. I'm not sure, because if not done carefully, the second pluck would damp the first, possibly making it uneven.
My two questions are: how is it played, and how is it notated?
I was very impressed with how effective and well executed this effect was.