No one's claiming that Rawat says he's the only incarnation of God to ever live.  Just that he's one, that's all. 

And before you try to say that maybe he claims there are all sorts of Perfect Masters at any one time, read this:

Wood: Do you think that there is only one Perfect Master? 

Maharaj Ji: You see –here I want to be very frank –people come to me and ask me about this, and they say, “What is your opinion about a Perfect Master? Is there one, is there two?” I tell them my opinion that there is only one Perfect Master. Because perfectness, is one, not two, not three. So there is only one Perfect Master in this world. And because he is perfect, that’s it. He is perfect. You just can’t divide perfect.

So it's pretty simple:  one Perfect Master in the world at a time and he just happens to be God.