Yes, I have seen the results - not pretty (two heads or several legs, etc.). More often than not, the egg doesn"t hatch and I never did see any survive over a day or so.
Years ago, we had a couple of chickens that would lay double-egged yolks about once every two weeks. Occasionally, one of them would lay a three-yolk egg. Our mother would never allow us to try and hatch either the two or the three-yolked ones. She said that was a cruel thing to do even something as dumb as a chicken.