When a new, conquering people came into a district for the first time, they would ask the local people (who spoke a different language): What is that? and point to the local river, mountain etc. And instead of giving the name, the locals would usually say: It's a river. Anyone would. But the new, powerful people usually couldn't be bothered to learn a whole new language so they'd just hear the word for river and think it was the name of that special one. So you often get names that actually mean : the river River, with the word in two languages. Hope that helps!