Doubling up is the practice of using multiple versions of the same part in order to add texture, thickness or in this case, enhanced stereo width. I talk a lot about the importance of energy in a piece of music and I believe that this process really adds energy.
I talk about doubling up in the context of lead hook lines but also kick drums. I also go in to a little more detail here regarding layering kick drums.
NB If you take an identical sample, duplicate it, pan one hard left and one hard right, you do NOT get increased stereo width. Because the samples are identical you simply get a mono signal. If however you record the same thing twice and pan them, each recording will be sufficiently different, thanks to the laws of physics, to give you a stereo effect.