RE: prevent a chatbot from asking a question it knows

What do you mean by "already knows"?

In general in AIML if you define a response for something more specific, that response will be used.
i.e. if your bot had a response for "do you like apples" as "Yes." then it would give that response.
AIML does not support knowledge, only patterns and templates.

If you are using Self, you can store and use complex knowledge, but this requires programming experience.