my sil was pregnant with twins and while she had several ultrasounds throughout her pregnancy they did not pick up the second twin because her was mirroring his brother. so i would say anthing is possible.she found out when she went into labor.
ultrasound is pretty darn good these days.
if they only see one baby at this point, there is really no where for a second to be hiding.
By the time you are 15 weeks, an experienced ultrasound technician would be able to see twins if you are carrying them. Good luck!
Anything is possible, but it is more then likely that an ultrasound would have seen a second baby if there was one. Even if it hadn't been discovered yet, I am confident it would be prior to the end of your pregnancy. If you really think there might be two, request another ultrasound...those machines are very sensitive, and although anything is possible, I think it highly unlikely it would miss it, especially at this stage. Good luck!