(MC 1.16.1, MetalBarrels 3.2b)

There seems to be a duplication exploit where when an almost full double chest is upgraded to an Iron Chests iron chest using a Wood To Iron Barrel Upgrade (which is intended functionality from what I can tell?), the "right" half of the chest retains all of the original chest's contents rather than just the "lower half" of items. The "left" half of the chest will still contain whatever was in the "upper half" of the original double chest, effectively duplicating that half of the original items.