The point of stacking images is to improve the signal to noise ratio. If you take 20 images the noise will be different in each one so the stacking program can assume that features that appear in all images are signal and the rest is noise (put very simply). If you just duplicated a single image then the noise would be the same in all and there would be no benefit to stacking.
They describe the use of the different types of images for stacking here:
http://deepskystacker.free.fr/english/faq.htm