>>49248753there are no one of a kind mons
going by the delta episode, hoopa's portals and usum, there are multiple parallel worlds and dimensions which can interact provided certain conditions are met
in usum you fight a different worlds version of many box legendaries which are "unique" for every world (kyogre, xerneas, zekrom)
That being said, there are some lehendaries which aren't unique even within a single world. Minor legends like the bird and beast trio, the eon duo, the regis and others can be found multiple times, albeit they are extremely rare (frontier brains in particular use them in canon). Mons like arceus, dialga, palkia, weather trio, Tao trio and so on are most likely unique within their own worlds.
