There is a Bug that can duplicate items in a barrel...
when a player opens a barrel's GUI, then another player breaks the barrel, it occurs...

the first player still have the GUI, but the second player can pick up all the items dropped from the barrel

Try removing the items from the GUI, they might be "ghost" items and disappear after MC realizes that those items were dropped/ picked up already

You can retest it... I remember that double of the items could be combined together... So it's a better way that closing the GUI when the block is broken by others. Or set it to unbreakable when it's opening.

Figured it out ty for reporting