comment_exists( string $comment_author, string $comment_date, string $timezone = 'blog' )

Source

File: wp-admin/includes/comment.php

function comment_exists( $comment_author, $comment_date, $timezone = 'blog' ) {
	global $wpdb;

	$date_field = 'comment_date';
	if ( 'gmt' === $timezone ) {
		$date_field = 'comment_date_gmt';
	}

	return $wpdb->get_var(
		$wpdb->prepare(
			"SELECT comment_post_ID FROM $wpdb->comments
			WHERE comment_author = %s AND $date_field = %s",
			stripslashes( $comment_author ),
			stripslashes( $comment_date )
		)
	);
}