How did the body double in jojo's Wonderful Adventure come into being?

According to Araki himself, it's an out-of-body experience, so it's called stand, body double because I'm standing by your side. There are three main reasons: 1. When the vitality and spirit are strong enough, the body body double will be automatically generated and will accompany you from birth. 2. Dior was resurrected with the body of Jonathan of Yostad family, so everyone in Yostad family has similar abilities. 3. Survivors who are shot by the mysterious Egyptian "bow and arrow" will gain twice the physical ability.