Unless it's cold outside then I can't see any reason to use double glazing. It may even cut down on your heat transmission. On the other hand I could be wrong it's been a long time since I worked with solar panels.
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Redesign has it right - you want maximum light transmittal and good heat (infrared) insulation. Light goes into the solar heater, heats stuff up, and is radiated off the black objects as infrared heat. Infrared bounces off glass, thus a greenhouse is made. Double glazing reduces the initial input slightly, but would help insulate from heat loss in a cold environment. 6.5 of one, half a bakers dozen of the other.
Many thanks for your comments.