ultra sound , yoke sack , very tiny figure

This is my 8 weeks pregnancy. I had an ultra sound pictures taken by doctor's assistant. The picture clearly shows the baby and it's heart beat. on the top of the picture, there we see another small circle like a yoke sack and a very tiny figure which is not equal to the 8 weeks baby. I am confused of what it was actually. the Medical assitant says, it could be any fibre growth or it may be another undeveloped fetus. can any one clarify of what it is? your answers would be helpful for me.
The med assistant have given another appointment for 10th week ultra sound, which is on July 23. Should i wait until next appointment or can i clarify it with my doctor during this week itself? Kindly advice.