When shooting athletes in motion with my DF+ and IQ250 or my newer XF and IQ3-100, I get image artifacts that look a bit like a double exposure. This only happens when flash is used. I have used ProFoto B1 units and the Pro-8 and Pro-10 power packs with pro heads. At first I thought it was motion blur, but on closer inspection, it is more like two images that are just slightly off-registered. This happens at any shutter speed up to and including the maximum speed of 1/1600s.
I thought this happened because the ambient daylight was sufficient to light the subject at 1/1600s but the flash had a shorter duration, thus creating a second exposure within the 1/1600s main exposure. However, this also happens in dark rooms lit entirely by flash, and I have since discovered that the B1 units I used outside are slower at full power than 1/1600s.
Any ideas how to stop this from happening?
Attached is a tiny full-size crop from one of these images.