Follow these instuctions:
• Make sure your server supports plugins (Paper/Spigot will do). If you have to change it, restart your server.
• Download the skript plugins from their github page. It's a .jar file. 2.6+ should do. Restart your server again.
• Go to your plugins file and open the skript file, then go to scripts. You can delete all the files there because you most likely won't need it.
• Create a new file called "Dupe.sk"
• Open the file and add the script I sent below
• Save the file
• In your minecraft server, type "/sk reload Dupe.sk" (This is case sensitive!)
• Type /dupe, it should dupe your item!
command /dupe:
trigger:
give player player's tool