Custom Object Menu Item duplicates

Summary

stopping duplicates Menu Items being added

Content

Hello,

 we have a custom menu object  for course codes, the table lists the unique id as   1-J2V, 2-22P etc which seems to be  the number at the front is the order it was entered appended to the actual code.  just the code i.e J2V appears on the menu

 Problem is that there is noting stopping a user from entering the same code multiple times it will just create like this   (1-J2V, 2-J2V, 3-J2V etc etc)

we need to find  any duplicates  before importing them in excel to avoid this. The course code is also linked to another table "course_catalogue"