The only way to deal with this requires use of FileMaker Pro Advanced, which allows you to set up Custom Menus.
That will give you two options:
1) Create a Custom Menu set that removes the Duplicate Record command, or
2) Create a Duplicate Record script that asks the user to Confirm (or Cancel) before duplication and exits on Cancel. You can then create a Custom Menu set that replaces the standard Duplicate Record command with the script. You can even add some smarts to the script to test for table or layout name to avoid creating duplicates where it makes no sense to do so.
Commonly known as a 'wetware' problem.
Unfortunately we developers have to take on board every potential weakness of human beings when designing our solutions...
Rob's advice is spot on. I like to control these things so that they are button-driven and there is no need for accessing menus... Lock everything up so they can only do what you provide functionality to do and nothing more... is my general advice.