My two sons had a vanishing twin. I had a twin in the womb too.
I know that feeling of loss though.
The cool(?) thing is, often your baby will have a birthmark - their twin. It is different DNA than the rest of them. I find this kinda intriguing.
My oldest son likes to say he is a born winner, that he won in the womb and absorbed his twin.
I often wonder why I lost a twin with each boy. If i could have carried the twins had I done something different. What the twin would be like, etc.
This is normal for any loss. Don't linger, celebrate your healthy baby growing inside you. Get support if it does weigh on you.