Mt. Rushmore, why is there two?

I was doing a search on national monuments and I came across Mt. Rushmore. I did a google maps search and got Portland OR. Of course at first I was skeptical and so I did a street view. Sure enough there it was. but then Later I found the same thing in Keystone, SD. Which one is the real one? and if they're both National monuments. Why do we have two?