Twins or no?

I had this sonogram this past Thursday. We all seen what looked like 2 heartbeats but they were only able to pick one up and it was 176bpm. I'm going this Thursday to see if a definitive decision can be made on if I'm having twins. This is my 4th pregnancy and I have 3 kids and I have never seen an ultrasound quite like this. Has this ever happened to anyone? One measured at 6w 6d and the other at 6w 3d.