Twin pregnancy

So at 6 weeks there was 2 sacs & yolks. And HCG was 8,000+

Now fast forward to 13 weeks and there’s only 1 sac & baby with toes fingers and even moving.

The 2nd sac disappeared and no baby.

Is this common in early pregnancy with twins? For one sac to vanish?

Or could it have been a mistake? There was always 1 sac?