Derren said:
Why are you not allowed to make a history check to know something like that? And how does this information allow you to make an additional history check?
A history check may well tell you that "the Visier of Alfric DID indeed cast some important Oracle Spells, and sent them along as warnings that helped him in a timely fashion, so the King DID send aid as promised."
It doesn't tell you that the Duke actually cares about this fact, and it especially doesn't tell you that the Duke actually has his facts wrong and believes that Alfric sent no aid at all. In other words, without the lead-in, knowing which historical insights are likely to cause the Duke to aid you instead of hinder you will be, in this case, not possible.
Moreover, if you bring up the right fact in the wrong context, you could well do more harm than good to your cause.
Derren said:
Also what happens when the wording of the first diplomacy check did not allow for this answer, what then? Can the PCs still make a history check or not?
The wording of the first diplomacy check doesn't stop the Duke from getting to say some words that reveal his interest in a particular historical topic, as well as some of his perspective on the topic so you don't stick your foot in your mouth when you pontificate on the subject.
EDIT: None of this is to say that using history in negotiations should be
generally disallowed. But in at least some circumstances it makes sense to hold it back until the PCs have some ideas on how to
apply the skill rather than just "use" it.