Removing or prevent duplications of entries

How do you remove duplications?

Basically, a user enters an item and can keep reentering the same item. How do I prevent this? I can show using formulas that an item is a duplicate but I can not say pick the last occurrence or the first occurrence of the duplicated item and ignore the others.

app link: https://berserk-quilt-3114.glideapp.io/

spreadsheet: https://docs.google.com/spreadsheets/d/1spUpm4u8tNY9tULRAchPMeQFZ-5ZYsbCcJJOnJm02rI/edit?usp=sharing

Any solution?

Here is a use case app’

One way is to have a 2nd sheet to store user submission, check for duplicate, if duplicate found, return a message to user “record exist” , if not duplicate, add it to final sheet.