The sickle-cell mutation, HbS, is found on five main haploptypes, and for a long time there has been argument as to whether this mutation originated once, or multiple times. A new paper indicates this it happened only once, in Africa, about 7300 years ago, even though you can now find versions in places as far away as India. There’s a recombination hotspot near the gene which apparently shuffled the haplotypes and made it seem as if the mutation had occurred repeatedly.
I’m not sure if people have talked about this (someone may have, it being a much-discussed topic) but there was always a reason to suspect a single origin: it never happened in Southeast Asia or Melanesia, even though there is plenty of malaria in the region, and lots of local genetic defenses like Hemoglobin E in Thailand and Melanesian ovalocytosis. But no sickle-cell. I can think of no case in which the same malaria-protective mutation is seen in Africa or is environs and Southeast Asia or Melanesia. Different mutations of the same gene, sometimes, but never the same mutation.