Index: src/wp-includes/query.php
===================================================================
--- src/wp-includes/query.php	(revision 35953)
+++ src/wp-includes/query.php	(working copy)
@@ -4978,6 +4978,11 @@
 		if ( is_post_type_hierarchical( $post_type ) )
 			return;
 
+		// If the post already exists, we shouldn't try to redirect
+		if ( get_posts( array( 'name' => $wp_query->query_vars['name'], 'post_type' => $post_type ) ) ) {
+			return;
+		}
+
 		$query = $wpdb->prepare("SELECT post_id FROM $wpdb->postmeta, $wpdb->posts WHERE ID = post_id AND post_type = %s AND meta_key = '_wp_old_slug' AND meta_value = %s", $post_type, $wp_query->query_vars['name']);
 
 		// if year, monthnum, or day have been specified, make our query more precise