Yep Dopplers are notoriously unreliable, evern OB's have trouble using them. Trying to figure out which heartbeat is which.
Try not to stress PZ, as Junglemum has suggested, perhaps see if you can pop down to either Sunshine or Footscray and explain your situation, they may be able to do a quick scan for you to ease your mind
I remember my OB using a hand held doppler very late in the pregnancy and was having trouble working out my heartbeat or babies heartbeat, he then took me to a room where he had a little scan machine and all was just fine.