Luffy will punch DD's glasses off as soon as he breaks in that castle. Luffy only fights 1v1 and its been shown alot of times that Luffy doesn't like it when people get involved in other peoples fights even if his own nakama is clearly gonna lose (Zoro vs Mihawk for example). So im thinking that either
1. Luffy will let Law 1v1 DD and DD will beat Law and Luffy comes in to take care of the rest (similar to Zoro vs Arlong)
2. Luffy will punch DD's glasses off and fight him...it'll be a good fight but Luffy will lose but DD will be weakened alot and when DD is about to finish him off we find out that Law has uncuffed himself and then it'll be a weakened Law vs a weakened DD. Law will finish him off.
3. Luffy beats DD and leaves him near death but Law will kill him
Anyways i think Law will play a decisive role in this fight and it wont be 2v1.