To dupe items, one must time their actions perfectly.
Step 1: Wait for server restart warnings.
Step 2: The last 1-2 minutes, put all your stuff into a storage container. (Have tested with v3s cargo trucks and flatbeds)
Step 3: Wait for the server restart
Step 4: ???
Step 5: You have succesfully duped the items you put into the container.
This only works if timed properly and I have tried it on the Dayz Underground servers.