All sorts of marriage taboos were worked out to avoid this. Also, I think that the 'raids' (to steal women) and fighting between tribes was a way to expand the gene pool... even though primitive people didn't comprehend DNA.
They may not have understood DNA, but they knew which would make desirable brides—provided, of course, they became assimilated. I think many American Indian tribes held captives as slaves for a time, and if they worked out, they could move up in status. If not, they stayed slaves, or...well, worms need to eat, too.