#5. "RE: New D5100 shutter fires twice.....?" In response to Reply # 3
What exposure mode are you using? Auto, P, S, A, M, or one of the Vari-Program (Pictogram) modes?
Are you shooting indoors in low light?
In low light, in P, S, A, and some of the Vari-Program modes the camera will choose a slow shutter speed. If you deploy the built in Speedlight the shutter speed should increase. You could also increase the ISO.
In Auto exposure mode, the built in Speedlight should deploy whenever the light is low enough that the exposure will be slow enough to capture a blurred image due to camera shake. Good Luck and Enjoy your Nikons!