--- a/src/internet-node/arp-cache.cc	Mon Jul 16 22:49:48 2007 -0700
+++ b/src/internet-node/arp-cache.cc	Wed Jul 18 11:43:39 2007 +0100
@@ -109,6 +109,8 @@
 ArpCache::Entry *
 ArpCache::Add (Ipv4Address to)
 {
+  NS_ASSERT (m_arpCache.find (to) == m_arpCache.end ());
+
   ArpCache::Entry *entry = new ArpCache::Entry (this);
   m_arpCache[to] = entry;  
   return entry;