diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index de0042ff32..0962da02f8 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -5077,7 +5077,9 @@ function wp_allowed_protocols() {
 
 	if ( empty( $protocols ) ) {
 		$protocols = array( 'http', 'https', 'ftp', 'ftps', 'mailto', 'news', 'irc', 'gopher', 'nntp', 'feed', 'telnet', 'mms', 'rtsp', 'svn', 'tel', 'fax', 'xmpp', 'webcal', 'urn' );
+	}
 
+	if ( ! did_action( 'wp_loaded' ) ) {
 		/**
 		 * Filters the list of protocols allowed in HTML attributes.
 		 *
@@ -5085,7 +5087,7 @@ function wp_allowed_protocols() {
 		 *
 		 * @param array $protocols Array of allowed protocols e.g. 'http', 'ftp', 'tel', and more.
 		 */
-		$protocols = apply_filters( 'kses_allowed_protocols', $protocols );
+		$protocols = array_unique( (array) apply_filters( 'kses_allowed_protocols', $protocols ) );
 	}
 
 	return $protocols;