If it’s a single key for the ultimate edition (rather than just the base game), then yeah it’ll activate for you just fine even if you already own part of the package.

just noting you can pretty much always redeem a key that gives more than what you own and it will activate the missing content.

there may be a few exceptions here and there, but in general that is how it works.

also to note, fanatical provides this sub for fallout NV ultimate: here

has no region locks, so if you desire to delete and recreate with no region locks you can, can also just leave it as is for another US person to be a guaranteed winner.