I just had two treasure chests stacked on top of each other with the exact same guardian and loot. I cleared the monsters around the chest and then stepped on it to open. I fought the guard, picked up the guards loot and then stepped on the chest to grab that item. While picking up that item I got another chest opening message and the same guardian spawned dropping identical loot to the first time the chest was opened.
Just a guess but this may have been caused by the Meteoric Crash prodigy landing on the tile with the chest while I was clearing the area before opening.http://te4.org/characters/14751/tome/fe ... 74ad=14751