Your hop up unit is having issues, either it cracked or has been positioned wrong. Either way the hop up rubber bucking moves forward enough that two BBs can enter the chamber instead of just one. Give it a good looksee over and see if anything wrong with it. I've fixed a half ozen guns with double feeding issue, all were related to hop up and thing else. I'm looking forward to seeing someone post a different reason from what they've found for this issue.
