Mercury's orbital speed changes throughout its orbit around the sun. For some locations on the surface of Mercury, when the rotation speed of Mercury drops below the speed of Mercury around the Sun, the Sun will rise and then after a few hours set, and then rise again to continue its motion across the sky. I do not know the dates for this phenomenon, but perhaps there are some planetaria simulators out there that would enable you to watch sunrise from Mercury's surface and by trial and error you could find the proper latitudes and times of the year when this happens. Sorry!