Index: src/wp-includes/load.php
===================================================================
--- src/wp-includes/load.php	(revision 27626)
+++ src/wp-includes/load.php	(working copy)
@@ -390,14 +390,15 @@
  * @since 3.0.0
  */
 function wp_start_object_cache() {
-	global $blog_id;
+	global $blog_id, $_wp_using_ext_object_cache;
 
 	$first_init = false;
  	if ( ! function_exists( 'wp_cache_init' ) ) {
 		if ( file_exists( WP_CONTENT_DIR . '/object-cache.php' ) ) {
 			require_once ( WP_CONTENT_DIR . '/object-cache.php' );
-			if ( function_exists( 'wp_cache_init' ) )
+			if ( function_exists( 'wp_cache_init' ) && ! isset( $_wp_using_ext_object_cache ) ) {
 				wp_using_ext_object_cache( true );
+			}
 		}
 
 		$first_init = true;