Index: src/wp-includes/taxonomy.php
===================================================================
--- src/wp-includes/taxonomy.php	(revision 25496)
+++ src/wp-includes/taxonomy.php	(working copy)
@@ -2089,9 +2089,16 @@
 	$name = wp_unslash($name);
 	$description = wp_unslash($description);
 
-	if ( empty($slug) )
-		$slug = sanitize_title($name);
+	$name = trim( $name );
 
+	if ( empty( $slug ) ) {
+		$existing_term = get_term_by( 'name', $name, $taxonomy );
+		if ( $existing_term )
+			$slug = $existing_term->slug;
+		else
+			$slug = sanitize_title( $name );
+	}
+
 	$term_group = 0;
 	if ( $alias_of ) {
 		$alias = $wpdb->get_row( $wpdb->prepare( "SELECT term_id, term_group FROM $wpdb->terms WHERE slug = %s", $alias_of) );