Ultrasound - saw two heartbeats but one baby

Hello, I had an ultrasound today. I am 7 weeks 4 days. This is the third ultrasound I've had due to being high risk and also having a subchorionic hematoma. At the ultrasound today, we saw baby and heard the heartbeat. This is the second time for both of those things. The baby is measuring 7+2 and has a heartrate of 144. Our doctor spent some extra time and said he had to pause because he saw a lot of pulsing going on. He wanted to count, and he did only see the one baby. He pointed out the second visible heartbeat to us at the top of the gestational sac, right above baby. He attributed this second heartbeat to my own. Is there anyway this is an error and there may be another baby? Just curious. After our history, I am so happy with one healthy baby, but my hcg levels were 20,000 at 5 weeks and my symptoms have all been more intense than with my previous pregnancy. If anyone has any opinions I would really appreciate getting to hear them!