Index: wp-includes/compat.php
===================================================================
--- wp-includes/compat.php	(revision 12478)
+++ wp-includes/compat.php	(working copy)
@@ -129,7 +129,9 @@
 		global $wp_json;
 
 		if ( !is_a($wp_json, 'Services_JSON') ) {
-			require_once( 'class-json.php' );
+			if ( !in_array('Services_JSON', get_declared_classes() ) ) 
+				require_once( 'class-json.php' );
+
 			$wp_json = new Services_JSON();
 		}
 
@@ -142,7 +144,9 @@
 		global $wp_json;
 
 		if ( !is_a($wp_json, 'Services_JSON') ) {
-			require_once( 'class-json.php' );
+			if ( !in_array('Services_JSON', get_declared_classes() ) ) 
+				require_once( 'class-json.php' );
+			
 			$wp_json = new Services_JSON();
 		}