My H.C.G levels did not double, they rose slowly and My doctor thought it was blighted ovum (Miscarriage).
Not all people double the same as others
They almost doubled in 48 hours. Why didn't the doctor wait until the 72 hours to test? I would have them rechecked.
Any time you have HCG in your system you are pregnant. Viability of the pregnancy is what's under question here. I would wait for another test to see how it's coming along. It seems like you caught this pregnancy pretty early, so I think there is a chance for the pregnancy to work. You do want to see numbers double from here on out.