Ideally hCG levels should not be used to date a pregnancy, since these numbers can vary so widely.
It is not necessarily the level that matters, but rather the change in the level.Since your levels have almost double in 48 hours suggest normal pregnancy.
But yes possibility of twins can't be ruled out.Still a ultrasound at 6 weeks may be more useful in detecting fetal heart sound.
I think a 6 weeks scan is the earliest that can give some useful information.
Before that not much significant information can be derived.A fetal heart beat can only confirm viability and you should wait till 6 weeks for first ante natal ultrasound.
Still i do think you should not give much significance to the numbers and avoid anxiety.