When cast, Arcane Armor correctly adjusts the unit's defense by 2 and takes up an enchantment slot.  Trying to cast the spell again causes a box to appear on screen that correctly informs you that spell is already in effect on that unit and can't be cast again.

Arcane Weaponry, however, can be cast repeatedly on the same unit (the sovereign, in my case), increasing attack by 2 each time.  This spell does not appear to  take up an enchantment slot like Arcane Armor does.  The description indicates that it should.