Not had this happen with either motor (Skywatcher or JMI) so not sure what has gone wrong to cause the double lines. Clutching a few straws here, seems there could be the possibility that distortion is somehow introduced between the collimator body and its support. How tight have you got the tension on the belt? Only need a small amount,just enough to allow belt to grip the collimator body and turn it but feels slack if you pinch belt either side of the cog. Have you used the white screws in the support to alleviate any possible side to side movement, lastly did you have to join the belt, if so have you watched the lines as the belt turns and noted where the join is (being as the belt will be slightly thicker in the region of the joint tension on collimator body will increase slightly).
Hope you get it sorted!