Vanishing twin syndrome

Velia

Has anyone had a vanishing twin! Am so confused about it. My 1st ultrasound the weren’t sure how far along I was. My 1st scan only had on sac and on baby that measured 5 weeks. I had hcg level done and tripled. I had my second scan on Monday and the have 2 sacs and on baby the other sac is empty. Has any one had this happen? And at a different scan had 2 babies? Or was it really only one?