Finale of Devastation
Oracle Text
Search your library and/or graveyard for a creature card with converted mana cost X or less and put it onto the battlefield. If you search your library this way, shuffle. If X is 10 or more, creatures you control get +X/+X and gain haste until end of turn.
I expect it just means you don't have to search your library if you don't want to. You don't get 2 creatures. It would be written very different if that was what was intended.
If it said "or" you would have to choose one or the other as the spell resolves, which can lead to feel bad situations, like if you choose to search your library only to find out the creature you wanted has been exiled face down with something like Gonti, Lord of Luxury but you have a creature in your graveyard that's better than whatever is left in your library.
That's the templating they've been using lately. Here's an example. Sun-Blessed Mount. And you certainly don't get two copies of the same Planeswalker with that card.
If it just said "or" rather than "and/or", you wouldn't be able to search both. Before looking, you'd have to declare which you're searching and once you've made that decision, you would have to choose a creature from the chosen location. Considering you can look at your graveyard anytime you want, this wouldn't be a huge problem, but it would result in people taking longer to think about what's in their deck before deciding which to search.
Also, it says "for a creature card with converted mana cost X or less and put it onto the battlefield". Had they intended for you to get 2 creatures, the word each and them would have been used.