Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 32321)
+++ src/wp-includes/user.php	(working copy)
@@ -1899,7 +1899,7 @@
 	 * check if current email and new email are the same, or not, and check `email_exists`
 	 * accordingly.
 	 */
-	if ( ( ! $update || ( ! empty( $old_user_data ) && $user_email !== $old_user_data->user_email ) )
+	if ( ( ! $update || ( ! empty( $old_user_data ) && strcasecmp( $user_email, $old_user_data->user_email ) !== 0 ) )
 		&& ! defined( 'WP_IMPORTING' )
 		&& email_exists( $user_email )
 	) {