diff --git a/wp-blog-header.php b/wp-blog-header.php
index 0f7118f3c1..7120010419 100644
--- a/wp-blog-header.php
+++ b/wp-blog-header.php
@@ -12,6 +12,7 @@ if ( !isset($wp_did_header) ) {
 	require_once( dirname(__FILE__) . '/wp-load.php' );
 
 	wp();
+//echo '<pre>';  print_r($wp_query); echo '</pre>'; exit;
 
 	require_once( ABSPATH . WPINC . '/template-loader.php' );
 
diff --git a/wp-includes/query.php b/wp-includes/query.php
index 756e5e5c52..2af594d47a 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -1546,6 +1546,7 @@ class WP_Query {
 		}
 
 		if ( !empty($q['category__in']) ) {
+			$q['category__in'] = array_unique( $q['category__in'] );
 			$tax_query[] = array(
 				'taxonomy' => 'category',
 				'terms' => $q['category__in'],
@@ -1555,6 +1556,7 @@ class WP_Query {
 		}
 
 		if ( !empty($q['category__not_in']) ) {
+			$q['category__not_in'] = array_unique( $q['category__not_in'] );
 			$tax_query[] = array(
 				'taxonomy' => 'category',
 				'terms' => $q['category__not_in'],