Gestational twins

Tunisia • Mother of 4 little princess & engaged 7.29.16
I went to the ER 10/29 and i was told i was 6wks there was a sac but no baby. On 11/9 i went back again and found out i an pregnant with twins one sac measures 7w 2d the other 8w 2d. In the 7wk sac they don't see anything at all. I guess im asking was anyone pregnant with twins and one was seen but not the other