Is there a simple fix I can do to the lightgun?
Nope.
A light gun will never work properly with a scan doubler.
All a light gun does is sends a signal to the computer when it detects the electron beam from the CRT passing by where you have it pointed to on the monitor. With a scan doubler attached, the electron beam is not going to be where the computer thinks it is, and you'll get a false reading.
