diff -r 2eebc8f7dca5 -r 17e36e175725 lemon/core.h
--- a/lemon/core.h	Tue Nov 01 08:24:30 2011 +0100
+++ b/lemon/core.h	Tue Dec 20 17:35:45 2011 +0100
@@ -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();
         }