Unless you use "The Void-Multiverce Theory".

Basically Each final fantasy taking place, not in a seperate world, but a seperate parallel deminsion that is connected by the void.

This is the basis theory for the "There's only one Gilgimesh" Paradox. That every Gilgimesh ever encountered is infact the same one that Exdeath cast into the void in FF5, just apearing slightly diffrent as to not stand compleatly out in the diffrrent worlds.