check for dupulicate

Hi, admin
You had said that using a BeforeAdd event to check for duplicates.
Could you give me some suggestions for this?
And I would like to take off recurrence on week/month/yearly.
Thank you <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=12283&image=1&table=forumtopics' class='bbc_emoticon' alt=':)' />