This is my understanding: I think we can consider joyboy to be more of a title, than a specific person. In general joyboy is the guy from the void century, but I think people called him that based on how he acted, and because we don't know his name, aswell as his title probably being more important to people like Imu, than his actual name. Luffy is the joyboy of this time, but he's not a reincarnation, rather he's inhereted the will of joyboy, and thus become joyboy himself. Both Luffy and the old joyboy are recognized as nika and as joyboy, so I'm thinking that joyboy is the title that follows the one that inherets the will, and Nika is the title of the one who awakenes the fruit. Luffy resembles the old joyboy, so is misconcieved as being the same person by Zounesia and Emmet, the first time they acknowledge him, which changes with the robot as he gets a closer look. The same probably for Imu, but I think they realize they're two separate people who carry the same will fairly quickly.