Courier gave me a letter that I already had

Those two letters have nothing to do with what you received from the courier.

If those letters are genuinely new, then by the time you opened them, a new quest will have popped up, telling you to search out the new word.

To confirm that, look in both your active and completed quest list and see if the location mention in both letters are in your quest log (they should both be there, at least one would be completed, and possibly both).

Those letters can be dumped after being read. If, for some odd reason, you want to keep a corpse around and not disappear, you can dump the letter in the corpse to preserve it.