Second Gestational Sack??

Annie • 23 years old. Diagnosed with PCOS & hypothyroidism. Married for 5 years. Ellie Rose 💕👶🏼 & Millie Jean 💕🤰🏼
So I went for my first ultrasound at six weeks this morning. I suppose because of my thyroid disease and PCOS they were wanting to see me earlier. 
Everything was peachy keen until the ultrasound lady said that she saw a SECOND gestational sack... but we couldn't find a heart beat. The first sack was great. You can see my little tadpole and hear a heartbeat. The ultrasound technician said it could be a twin or it could just have disappeared. Only time will tell. 
We go see my obgyn and he immediately says "You're having twins!" 
And my husband and I were like WHAT?!
He says that next week we should be able to tell if it's legit or not. 
Has anyone ever had a second gestational sack that ended up being a twin?!