FLDIGI Users Manual 4.0
Fldigi supports a generic but robust set of contest functions. In addition to serial-in and serial-out you can capture and transmit three exchange sequences unique to a specific contest. Enter the exchange you want to send for each of the three. You can force the RST in/out to always be 599. That seems to be a norm for many contests. When operating in a CW contest you can have fldigi send cut numbers, T for 0, N for nine.
The serial number can be set to use leading zeros. You can specify the starting number for the sequence and how many digits are sent, ie: 0024. Pressing Reset will set the starting number to the QSO logging serial out field. See Contest How To for more info.
You can check for duplicates by any combination of the specified named fields. You can also specify that the duplicate had to occur with a given time interval. Some VHF contests allow a duplicate CALL after a given time interval.