TCP retransmission and duplicated ack
 

I am using mandrake 9.2 as an ftp server providing downloading service. During a ftp session, I found that sometimes the TCP sender triggered packets retransmission when only received one duplicated ack. Isn't the standard saying that the fourth duplicated ack will triggered TCP retransmission? However, sometimes this only one duplicated ack did not make any impact on the TCP sender, which conforms to the standard. Could anybody throw some light on my confusion? Is it complex to modify this part of the TCP codes to make it conform to the standard? Thank you in advance.