is it possible??

Is it still possible with the technology today to still overlook twins on an ultrasound? Hubby and I just listened to our bean with a Doppler and swear we found two different heartbeats. It went from 160 to 180 in like 10 seconds. We heard the first, and then it went faint and then we heard it the second time. And it kept shifting from one to the other. And we're pretty sure about the HR's becuz we timed them multiple times. I should also add that I am now 13w1d and have had 4 transvag u/s by this time. And don't go back until 15w5d on the 22nd. My last u/s I was 10w5d. Just wondering if anyone has been in this situation. Ty in advance!