Hi!
HCG levels are very variable! I have seen levels all over the place. You definitely should see the sac at this value, and you have. The measurement of the sac would be useful, if it was more than 15 mm and there is still no yolk sac, something is not right. Rather than another HCG (or in addition to it) I would order a progesterone level. If the value is greater than 15 that is good. If you do have another HCG done, it most likely won't double since the value is already so high. It should continue to rise though. If it declines, that is bad news.
I hope everything works out for you!
Good luck!
Dr B