I suppose it depends on what you mean by "extended duty?"
What is the cause of the extension?
If it is an FTL approved variation the maximum period of available discretion is usually reduced as part of the terms of the variation.
In other cases, the commander may at his discretion and after taking note of the circumstances of other members of his/her crew extend an FDP beyond that normally permitted, provide he/she is satisfied the flight can be made safely. The extension shall be calculated according to what actually happens, not on what was planned to happen. An extension of 3 hours is the maximum permitted except in cases of emergency. If the FDP involves 2 or more sectors a maximum of 2 hours discretion may be excercised prior to the first and subsequent sectors, but this may be 3 hours prior to the start of a single sector flight or immediately prior to the last sector of a multi-sector flight.
There is no particular requirement to consult with other crewmembers, only to note the circumstances that may be applicable to those individuals with regards to an extension (for example prior rest and extensions etc.)