Expert Mode Treasure bags. pls help

[POOPthatPEES]

i just went to expert mode, killed the eater of worlds and skelletron. I got a treasure bag from each one and then duped them so i could get developers gear. I opened up like 1000 of each one and i didn't get anything, what's the problem?

 

[Psyme]

Only HM bosses will have a chance for developer stuff to appear in their treasure bags.

 

[cheesegobrrrrrr]

How do u dupe