Ticket #33804: 33804.2.diff

src/wp-admin/network/site-new.php

@@ -89 +89 @@
         $password = 'N/A';
         $user_id = email_exists($email);
         if ( !$user_id ) { // Create a new user with a random password
+                $user_id = username_exists( $domain );
+                if ( $user_id ) {
+                        wp_die( __( 'The domain or path entered conflicts with an existing username.' ) );
+                }
                 $password = wp_generate_password( 12, false );
                 $user_id = wpmu_create_user( $domain, $password, $email );
                 if ( false === $user_id )