Hiding twin/second heartbeat — helpppp

Has anyone ever experienced finding out later that you were having twins jnstead of one?? Or have you ever found two heartbeats ?? I recently started to use a Doppler I’m 17 weeks and haven’t had an ultrasound since I was 7 weeks. I’ve been able to find two distinct heartbeats in two different places with two different rates. One is 150s and the other is 120s. It’s too high to be mine because I can also find mine which is in the 90s. What could this second heart beat be ?