A galaxy as the source of gravitational lensing is statistically expected to create 2 images (pairs) in average, while the cosmic strings would give you something in between 9 and 200; note that 11 belongs to this interval. ;-) The main skeptics' explanation is that the double images are just random pairs of galaxies etc. that happen to be similar to each other. Incidentally, I also recommend you an Italian page about CSL-1 and why they believe it is a double image caused by a cosmic string: