Originally posted by gifthorse The double exposure effect was simply (and unfortunately) the wind shaking the camera.
Ah! It's too easy to forget when looking at pictures: with lightning comes wind and rain which the camera had to endure. Using the intervalometer was a good idea. If you had a ND filter (like Na Horuk said) you could have had your shutter open almost all the time, instead of 1/6 of the time.
I'm anxiously awaiting my first monsoon season here so I can try this. I've got a few decent vantage points picked out.