I bought a simple SD card breakout board for a project, and it has a double row of pin headers, with each I/O having two pins (the I/O are GND, +5V, +3V, MISO, MOSI, SCK and CS). I hadn't seen this before and it got me thinking about why you would do this. The only reason I could come up with is if you wanted to chain multiple boards together, but that wouldn't make alot of sense in this case.
Could be just for more mechanical strength when soldered to a PCB. As such a module is subjected to some mechanical forces as users install and remove the SD chip, there would be much better resistance to bending forces with a duplicate set of header pins soldered to the board.