• FUN FACT #1: If during a mission you find a weapon that's already equipped, you can equip it to a different slot and hold the same weapon in duplicate (I did not try tri/quad wielding), something the inventory menu does not let you do on its own.
  
• FUN FACT #2: If two identical weapons are held at the same time, selecting twice the weapon in the upper slot will give it the ammo count of the same weapon held in the lower slot.
  
• FUN FACT #3: I <3 nuclear weapons

Basically, so long as the lower duplicates isn't expended, you can reload similar weapons in the upper slots infinitely, just open the inventory, select the upper duplicated weapon slot and double tap spacebar.

Now let's demonstrate this simple method using rocket nukes.