Ticket #32158: 32158.diff

src/wp-includes/user.php

@@ -1899 +1899 @@
          * 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 )
         ) {