How to do conditional publishing?

That’s because the hook “transition_post_status” is called:

1. After the post has been published or updated in the database
2. After the status has been updated in the database.

Based on your problem statement, I believe the hook you want is “pre_post_update”, as stated in relevant WordPress source code:

/**
 * Fires immediately before an existing post is updated in the database.
 *
 * @since 2.5.0
 *
 * @param int   $post_ID Post ID.
 * @param array $data    Array of unslashed post data.
 */
do_action( 'pre_post_update', $post_ID, $data );

This code is immediately followed by a call to $wpdb->update( $wpdb->posts, $data, $where ), which actually saves the post to the database.

Usability issue. I would advocate for not calling wp_die() if e.g. just the title is identical to one in the database. That’s a bad user experience. It would make more sense to use a add_filter to rename the title if this happened:

/**
 * Filters slashed post data just before it is inserted into the database.
 *
 * @since 2.7.0
 *
 * @param array $data    An array of slashed post data.
 * @param array $postarr An array of sanitized, but otherwise unmodified post data.
 */
$data = apply_filters( 'wp_insert_post_data', $data, $postarr );