It is possible that an increase in HCG levels from 900 to 15047 in just 9 days could indicate the presence of twins. However, it is important to note that HCG levels can vary greatly between pregnancies and individual women. The best way to confirm if you are carrying twins is through an ultrasound examination, which can detect multiple gestational sacs or fetal heartbeats. If you suspect you may be pregnant with twins, it is recommended to consult with your healthcare provider for further evaluation and guidance.