My first ultrasound was very early and they thought there might be two sacks. They were not sure, and had me plan to come back for another.
I remember being very stressed and worried between those two ultrasounds, not knowing if I could get excited or not.
My boys are 7. So, yes, it does happen sometimes.
We also had something similar. We had a very early unltrasound to determine a uterine pregnancy and they only saw one, at 8 weeks there were two hb. It can happen. Actually they thought that I had actually conceived one week apart because that's what they were measuring the whole pregnancy.