negative on doubling lines..
not from where I am from and the gliding I have done. Better to just replace any lines that are 6 months old (or more) or questionable.
There are some clever things that you can do to the lead lines that run from the bar and neat backups that you can do around the link points of the trimming system on the center lines, but really, when you are gliding and such, being high off the ground, you'll know by then what the gear can do.
It is pretty strong you know.
I have set up and made some very interesting tests on gear for kiting (lines, spreader bards, harness stitching, bar strength, etc) and pretty much all of it can handle the beyond hanging body weight that a kite-loop can produce. So gliding is really just your body weight under the kite (as to why redundancy (think foil bridles) and simple line checks are needed).
ill see about posting them here in a week or so. we'll see. The conclusions really came to quality gear is best, and it is all strong enough, IF people are very diligent to be checking lines and looking for FRAY! E.g., sk-75 a braided dyneema, when frayed about 50% of "pixels" running around the diameter of the cord, is considered roughly to be 60-65% weaker. So generally, replace anything that has a nick, chip, fuzzy spot, discoloration, stiff or softer feel from original, etc.