It depends. If it's clearly a same string, then using the same string makes translators work a bit nicer as they don't need to translate same thing many times. But if there's a change that in any language the content might be any different in two different places. Then creating a new is good.

Also new strings can be created later if need arises. And also other way round. Additional strings could be deleted, but that rarely happens in practice as no one is cleaning up so to say..