diff --git a/wp-admin/admin-ajax.php b/wp-admin/admin-ajax.php
index 5964218a7..2fd92eeb3 100644
--- a/wp-admin/admin-ajax.php
+++ b/wp-admin/admin-ajax.php
@@ -246,6 +246,14 @@ case 'add-cat' : // From Manage->Categories
 		$x->send();
 	}
 
+	if ( category_exists( trim( $_POST['cat_name'] ) ) ) {
+		$x = new WP_Ajax_Response( array(
+			'what' => 'cat',
+			'id' => new WP_Error( 'cat_exists', __('The category you are trying to create already exists.'), array( 'form-field' => 'cat_name' ) ),
+		) );
+		$x->send();
+	}
+	
 	$cat = wp_insert_category( $_POST, true );
 
 	if ( is_wp_error($cat) ) {
diff --git a/wp-includes/js/wp-ajax-response.js b/wp-includes/js/wp-ajax-response.js
index 419777371..292e9e950 100644
--- a/wp-includes/js/wp-ajax-response.js
+++ b/wp-includes/js/wp-ajax-response.js
@@ -50,8 +50,11 @@ wpAjax = jQuery.extend( {
 		else if ( 0 === x ) { return !re.html('<div class="error"><p>' + this.broken  + '</p></div>'); }
 		return true;
 	},
+	invalidateForm: function ( selector ) {
+		return jQuery( selector ).addClass( 'form-invalid' ).change( function() { jQuery(this).removeClass( 'form-invalid' ); } );
+	},
 	validateForm: function( selector ) {
 		selector = jQuery( selector );
-		return !selector.find('.form-required').andSelf().filter('.form-required:has(:input[value=""]), .form-required:input[value=""]').addClass( 'form-invalid' ).change( function() { jQuery(this).removeClass( 'form-invalid' ); } ).size();
+		return !wpAjax.invalidateForm( selector.find('.form-required').andSelf().filter('.form-required:has(:input[value=""]), .form-required:input[value=""]') ).size();
 	}
 }, wpAjax || { noPerm: 'You do not have permission to do that.', broken: 'AJAX is teh b0rked.' } );
diff --git a/wp-includes/script-loader.php b/wp-includes/script-loader.php
index 7181226ac..ea2ba9fd6 100644
--- a/wp-includes/script-loader.php
+++ b/wp-includes/script-loader.php
@@ -41,7 +41,7 @@ class WP_Scripts {
 
 		$this->add( 'prototype', '/wp-includes/js/prototype.js', false, '1.6');
 
-		$this->add( 'wp-ajax-response', '/wp-includes/js/wp-ajax-response.js', array('jquery'), '20080312' );
+		$this->add( 'wp-ajax-response', '/wp-includes/js/wp-ajax-response.js', array('jquery'), '20080316' );
 		$this->localize( 'wp-ajax-response', 'wpAjax', array(
 			'noPerm' => 'You do not have permission to do that.',
 			'broken' => 'AJAX is teh b0rked.'