Why does my flash on my camera do this?

This is just the internal flash im talking about. Only sometimes, when I use my flash, it "stobes" before it will take the picture. I cant ever get good shots when I use the flash because it goes off so many times before it will take the picture that whoever im photographing, makes funny faces because the flash is going off so many times before the photo is even taken. What is goin on here? I use a Canon xsi