Which hook do I use for the following scenarios?

Hi,

I am writing a plugin for my own site which allows user submitted content to a custom post type with custom fields.

I want to check for duplicates and have looked at two ways to do this.

1 – Run when a post is saved, to see if there is a duplicate in my post_meta table, if there is return false for wp_insert_post_data

however when I do this, it returns to the add post page for the custom post type with an error but still saves some data

2 – Save the post and once saved check for the duplicate and then delete the post and inform user of duplicate.

I can’t seem to find which action hook I would use for scenario 2.

Save_post wants a post id or post data, and I can’t see just how to use this.

My function should be very simple along the lines of

if ($post_type == 'media_link') {
		$query = "SELECT count(ID) FROM {$wpdb->prefix}media_links WHERE LOWER(link)='".strtolower($_POST["cctm_media_link"])."' and ";
		$count = $wpdb->get_var($query);
		if ($count > 1 ){
		 echo "there is a duplicate";
		}
else
{
echo "there are no duplicates";
}
		return $data;

So really I just need to run that query to see if there are other items, if there are then I need to do something with that like wp_delete()

also once I run the delete, how do I return a message to the user telling them that it was a duplicate?

Dan