• Plugin version Latest
• Minecraft version 1.17
• Server type Paper

Not sure if this is relevant or something fixable or even this plugins fault but
If you combine DeadChest with this plugin Portable Chests it creates the possibility to dupe, because if you place a container holding items next to an active deadchest it breaks the chest dropping the items however the chest also goes back into inventory holding the original items.

Fixed the issue from my plugin in the new release -> Portable Chests v1.4.0, you can find the details here