diff -r 949510b70df9 -r 9f22c22fe227 lemon/core.h
--- a/lemon/core.h	Fri Jun 25 05:48:27 2010 +0200
+++ b/lemon/core.h	Fri Jun 25 06:15:43 2010 +0200
@@ -394,6 +394,7 @@
       template <typename From, typename NodeRefMap, typename ArcRefMap>
       static void copy(const From& from, Digraph &to,
                        NodeRefMap& nodeRefMap, ArcRefMap& arcRefMap) {
+        to.clear();
         for (typename From::NodeIt it(from); it != INVALID; ++it) {
           nodeRefMap[it] = to.addNode();
         }
@@ -421,6 +422,7 @@
       template <typename From, typename NodeRefMap, typename EdgeRefMap>
       static void copy(const From& from, Graph &to,
                        NodeRefMap& nodeRefMap, EdgeRefMap& edgeRefMap) {
+        to.clear();
         for (typename From::NodeIt it(from); it != INVALID; ++it) {
           nodeRefMap[it] = to.addNode();
         }