I don't think it's that simple and I wouldn't like to just guess.
If the gun is picking up the leading edge of the current in the loop wire, the amount of dwell is added to the timing reading. It might look like double, bit that's basically just a fluke. It should really pick up on the falling edge of the loop wire to be accurate.
I would still not trust it, a plug lead between coil and plug is best option.