Dual fed simply means that the rotor is connected, via sliprings to a variable frequency 4 quadrant inverter, so what it can do, with the aid of a shaft position encoder, is vary the frequency and angular phasing of the rotor current so that the rotating magnetic field remains "synchronous". Well, not exactly synchronous, as it is still an induction generator, but "at or about synchronous speed" The reality is that the rotor can run at variable speeds, but the rotating magnetic field remains at ~1500 rpm
Enercon are different, in that they use a low speed generator without a gearbox. This requires all the power to be converted by a frequency converter at the bottom of the tower.
If there is an oversupply, a wind turbine will not back off simply because the frequency has increased, unless there is a specific control function required to that at that particular windfarm. All the ones we have worked on, where there is the ability to load manage, use external control signals, usually over leased lines direct from the electricity substations.
So its still very much a case by case basis, but I don;t think I have ever seen a windfarm that controls power by frequency.