Possible twins?!

I’m currently 16 weeks 6 days pregnant with our first and we’ve been told we are carrying One baby. According to the ultrasound. Well we purchased a fetal heart Doppler and at first we were only hearing one heartbeat and it was saying 1 heartbeat was picked up. Well now the last few times I have used it, it says 2 heartbeats and we have actually heard where it sounds as if there is two different heart beats. Both fast as each other but at different times. I know it won’t mine because I can put it at a certain spot and hear my own and of course it’s much slower than the baby’s heartbeat.

Has anyone ever had this happen or not find out you’re having twins until later in the pregnancy? This possible?