I looked at betabase.info myself at the beginning of this pregnancy and sent myself into hysterics with a phone call to my doc after getting my HCG's back wondering if I was having triplets. I am on Prometrium (Progesterone) and haven't had my Progesterone levels tested. I did have my HCG measured because I have had an ectopic pregnancy and 3 miscarriages.
My levels for this pregnancy were: 13dpo= 396 & 15dpo= 958
It has been confirmed now with 3 ultrasounds that I am only having one baby. I was quite in shock because 396 at 13 dpo is the highest I have had so early. Though my DD wasn't far behind at 14 dpo= 319 & 16 dpo= 759. And DS#2 was at 15dpo =396 & 17dpo= 1504.
I spent all my free time after getting my results and seeing that I was more than double the number that the average triplet pregnancy was at 13dpo, online looking at websites. I can't tell you how many sites I saw where women were posting their twin and triplet HCG's and mine were well above theirs. But my case definately shows that high HCG's don't always indicate twins or triplets.
I think since you have a history of twins, it's entirely possible that you could be having twins again. But I don't think that just based on the numbers alone, it means you are having them. When is your first ultrasound?