Despite using two P-clips one of mine moved slightly when I very first installed them and the plastic coating rubbed on my tyre, so had to move the clips about to get the line to sit right. (The lines tend to lift themselves upwards or downwards depending on how you route it) I left it for a few months before replacing the damaged one and had no problems.
As I only had one line that needed replacing I spoke to HEL to see if I could buy a single one, they immediately sent me a whole front set free of charge, no questions asked!!