diff --git a/wp-includes/query.php b/wp-includes/query.php
index 663b1c9135..3389232e08 100644
--- a/wp-includes/query.php
+++ b/wp-includes/query.php
@@ -1654,6 +1654,9 @@ class WP_Query {
 		}
 
 		foreach ( $GLOBALS['wp_taxonomies'] as $taxonomy => $t ) {
+			if ( 'post_tag' == $taxonomy )
+				continue;	// Handled further down in the $q['tag'] block
+
 			if ( $t->query_var && !empty( $q[$t->query_var] ) ) {
 				$tax_query_defaults = array(
 					'taxonomy' => $taxonomy,
@@ -1790,7 +1793,7 @@ class WP_Query {
 		}
 
 		if ( !empty($q['tag_slug__in']) ) {
-			$q['tag_slug__in'] = array_map('sanitize_title', (array) $q['tag_slug__in']);
+			$q['tag_slug__in'] = array_map('sanitize_title', array_unique( (array) $q['tag_slug__in'] ) );
 			$tax_query[] = array(
 				'taxonomy' => 'post_tag',
 				'terms' => $q['tag_slug__in'],
@@ -1799,7 +1802,7 @@ class WP_Query {
 		}
 
 		if ( !empty($q['tag_slug__and']) ) {
-			$q['tag_slug__and'] = array_map('sanitize_title', (array) $q['tag_slug__and']);
+			$q['tag_slug__and'] = array_map('sanitize_title', array_unique( (array) $q['tag_slug__and'] ) );
 			$tax_query[] = array(
 				'taxonomy' => 'post_tag',
 				'terms' => $q['tag_slug__and'],