We are having FTP performance problems when taking a certain IP route
through our network.

A packet capture shows an Active FTP session riddled with tons of TCP
Duplicate Acks by the receiver and subsequent Retransmissions by the
sender (CCC).... no wonder the user experience was very poor. However,
although we have been able to identify the symptoms, the root cause has
eluded us. Our analysis shows that the sender appears to be properly
transmitting his FTP Data packets, but the receiver is sending a normal
ACK followed immediately by a DUP ACK. This ongoing "paranoid" behavior
by the receiver results in Retransmissions by the sender, thus clogging
up the flow with a snowballing effect.

The strange thing here is that according to the timestamps, there is no
need for the receiver to be sending a DUP ACK because there appears to
be no delay whatsoever. So what else would cause the receiver to feel
the need to send a DUP ACK if there was no obvious delay ?