I think Old Smokey is correct.
I am not familiar with the 777, but 737, and I ""assume" similarities.
Your Derate and assumed temp is set for your departure runway. Say you derate.
When you go land a bit later somewhere else, and make a go-around, you might need full go-around trust due to weight and obstacles. So full go around trust is available here by pushing TOGA twice. This trust can not be a function of the derate you set at your departure airport, right?
So push TOGA again after TOGA is set for take off, gives you full rated trust.
It might give you even so much power that , in case of an engine failure, you cannot maintain directional control, due to the fact that your speeds were calculated for the Derated trust,and now you have increased VMCG and VMCA.
The FCTM warns you for this in the B737.