Possible twins?

I am 13 weeks and 1 day. The last ultrasound I had was at 11 weeks 4 days. I have been listening to baby’s heartbeat on my home Doppler and every now and then I hear what sounds like an overlapping heartbeat going at the same speed, but on ultrasounds I’ve only ever seen one baby. Twins run in my family, and frankly I’m surprised that I’m not pregnant with twins. I’ve had three people so far tell me that I could have twins and the other one is just hiding, which sounds stupid. So is it possible that I could be having twins and everyone just missed the second one?