doc: fix `duplicate` example

PR Summary

Introduction of 'Duplicate' Method in the Collection Class A new function has been introduced allowing for the identification and return of duplicates in a collection, predicated on particular criteria. This is an incredibly valuable tool providing a streamlined approach to locating repetitious entries in our database. Actions such as identifying multiple users, people, or even cat objects with the same names can be quickly executed using this new method. This reduces redundancy and improves overall system efficiency.