Sorry for your lose! But I'm glad it was only a duck! I thought they stopped making all continuous chargers by Law years ago and went to only pulse ones for Safety.
We lost calves to that kind of charger years ago. I never tested this but, I have heard if a child/ adult grabed the hot wire by accident - they won't be able to open their hand to stop being zapped with a continuous charge. If someone else tries to pull the person away from the fence - they then get zapped too. The only way to free a person or animal is shut off the charger or knock the person/animal off the fence with a wood board or dry stick. I would be getting rid of that charger yesterday!!!
So sorry for your loss. That's a major bummer. I think I might be going over to the store to have a chat with the manager about what constitutes "safe for poultry usage".
I hope you get some babies to replace the gander. Small comfort, I know.