Given that their last common ancestor could have been many generations ago, and that the system was entirely patrilineal so it was legal to marry close maternal relatives, it was a pretty ridiculous system - but not quite as ridiculous as not being able to marry someone with the same surname in a country where most of the population is a Kim, Lee, or Park.
(5057 vs 609 according to one site)
Khan is a common surname particularly among Muslims of Central Asian and South Asian origin. It is one of the most common surnames in the world, shared by over 12 million people in Asia and 24 million worldwide. It is the surname of over 108,674 British Asians, making it the 12th most common surname in the United Kingdom.