--- a/src/internet/model/tcp-socket-base.cc	Fri Sep 14 22:07:40 2012 +0200
+++ b/src/internet/model/tcp-socket-base.cc	Thu Sep 20 10:13:28 2012 -0400
@@ -1028,7 +1028,7 @@
     }
   else if (tcpHeader.GetAckNumber () == m_txBuffer.HeadSequence ())
     { // Case 2: Potentially a duplicated ACK
-      if (tcpHeader.GetAckNumber () < m_nextTxSequence)
+      if (tcpHeader.GetAckNumber () < m_nextTxSequence && packet->GetSize() == 0)
         {
           NS_LOG_LOGIC ("Dupack of " << tcpHeader.GetAckNumber ());
           DupAck (tcpHeader, ++m_dupAckCount);