There are two duplicate locks in the game, that I know of (and the armoury door isn't one of them).
The key you are given to Nericon's Garden by Blake can also open the castle graveyard (which would be opened anyway after going to the castle and preceding with the main plot quest for awhile). There is another key that can open either lock in a back room of Walker's house (the alchemist in Ars Magicana), though it is a little tricky getting in there without upsetting him.
