There are multiple other people named "Jack Garzella", i.e. that have the exact same first and last name as me. Thus, for as long as I can remember everyone has called me "JJ", which comes from my first and middle initials.
If you're referring to me in a formal context (for example, a citation), please use my full name with middle initial, i.e. "Jack J. Garzella" or "Jack J Garzella". If the context is informal (for example, talking in person), feel free to call me JJ!