By "picking up" a chest with dolly added by the mod, all the contents come out as items you can pick up. Then when the chest is re-placed the contents of the chest are still in there. You can then take them out or put the duped items back in.

That should, and does not, happen in my dev environment with just Forge and EnU. Something must be changing how one of the fields in the World class works.

I need more information about the pack, MC version etc. Is this on a server, or otherwise using for example Sponge, or maybe some protection mods? Was it a vanilla chest, or what type?