This bug was discovered in version 4.0-460. I discovered it while playing the most recent release of the Better Minecraft 4 (v36) modpack.

• Obtain Flamarang/Pickarang/etc
• Fill every inventory slot except for one
  • Empty slot for rang
  • You can put 1 cobblestone in every slot
• Find bedrock (or any other unbreakable block)
• Right-click and throw your rang at it
• Your rang should return to your inventory, but a new rang will also have been tossed onto the ground
• Free up a single item slot in your inventory
• You should pick up the duplicated rang
• You now have two rangs
• This can be repeated infinitely as long as you're throwing it at an unbreakable block while having a full inventory.

To follow-up:

• This ONLY works on unbreakable blocks like bedrock or reinforced deep slate. It doesn't work on blocks that just cannot simply be broken by a rank (i.e.: obsidian, ancient debris, etc.)