No there is no way to stack any weaponry or armor because if you buy two of the same armor or weapon or pick up two of the same weapon or armor it will just discard them. Only things that can be stacked are Arcane Spells,Celestial Spells,Nether Spells,Potions,and Scrolls.
This is the purpose of the 'junk' tab - to store one extra of things you collect. I don't know about anyone else, but by the end of the game, I had a very loaded up inventory without even being able to get duplicates.
You don't, right now. It's bugged. The developers have said as much. It's also one of the fixes slated for the 1.2 patch, supposedly due out by the end of the week.
To the Righteous belong the fruits of violent victory. The rest of us will have to settle for warm friends, warm lovers, and a wink from a quietly supportive universe.