There have always been many people named Jacob, and most of them will be called shorter versions-Jake, extended Jack, Jacky and so on.
Just like Joseph (Yue Se) named Joe, Joe and Joey.
Benjamin (Ben Ming Jie) is called Ben and Benny.
T-Bag's name is Ted Teddy, and sometimes he may be called Teddy Bear in a joke.
It's a nickname, just like some people will call their family members, Xiaoming, Amin or Mingzai, or even reduplicated words, such as Mingming. European names have their own extensions from nicknames and different languages. It's hard to do it overnight, so just get to know it.
Never heard of the double alphabet-usually they are called Jamie and Zoe, which is more in line with the phonetic principle.