Had ultrasound at 6. 2 weeks one had heartbeat 122. Dr said was a twin other sac in ultrasound will I expel it empty sac or does it go away?

Dissolves. Commonly twin gestation turns into single one due to loss of one fetus. As long as this happens early, there is usually no consequences for the remaining fetus. The other one is reabsorbed. Usually this happens for a good reason, and may mean one is not healthy. You could not have prevented it or caused it either. Get followup U/S to make sure you baby is growing well inside you. Best wishes !!
Vanishes. It is not terribly uncommon to see a nonviable twin on ultrasound in association with an early pregnancy. That nonviable twin will be compressed and reabsorbed as the viable pregnancy grows and develops. It is referred to as a "vanishing twin".
Go away. Usually when there are twin sacs and only one develops, the body may absorb it. Ocasionally the obstetrician will take care of the sac.