Jack,
I cannot see your attachment,but still baffled as to what could be causing the doubling of the line. I have not seen the effect in my setup at all. The only thing I can think of once again is that the belt is too tight and placing the lens slightly out of line,maybe my using a much thinner belt places less pressure on the setup. Have you tried using a replacing the belt with a rubber band? Maybe the velcro round the collimator body is the problem (I have just placed belt round the body and have no slippage).
Regards
Alun