ChatTriggers mod is required.
This module allows you to check if your item is duped or not.
Run "/sdc check" while holding an item you want to check. A message will appear in chat.
You can download Seba Dupe Checker by clicking here.
1. Download the module
2. Press WINDOWS + R and type %appdata%
3. Go into .minecraft, then config, then ChatTriggers and then put the file in modules folder
4. You are done!