Back in the day, there was a DM guideline/rule: always give the monster an even break.
I suggest that player race foes (Duergar, for example) should be allowed equal weaponry (repeating crossbows, runearms, for example) and equal training on that weaponry. Champions and bosses only, perhaps. Plus, that weaponry would drop (%) in that foe's remnant chest.
Imagine a red crown Beast Mark with vorpal repeater.