Problem:
The user would like to allow users to create posts, from the frontend, with unique titles. He uses custom code for validation, but the custom code fails for updates as the new title equals the old(existing) one.

Solution:
Introduce a check in the code that will handle the update case. Check reply https://toolset.com/forums/topic/issue-with-duplicate-post-titles-when-editing/#post-1823283

Relevant Documentation:
https://toolset.com/documentation/programmer-reference/cred-api/