In hero-marketplace I can buy an extra "Tlato Mosi Pakwa III", in my walid walkers game..

I've seen this once before, where a hero has a duplicate in the marketplace, or where it's even possible to buy a hero unlocked by a faction quest (wild walkers also).

First heroes of every race can appear in marketplace. It is not a bug. They can also be sold to marketplace, they are not unsellable. But to my knowledge, no hero obtained by faction quests can be sold to marketplace, neither they appear there. So everything is working fine and as ittended.