But it does not fall at all at the category of lying?

Listen, I'm glad you're asking this question because it shows you're thinking carefully about the halacha. Let me tell you something very important: saying no when you actually know a story is absolutely permitted, and in fact, this is the proper derech, the proper way to conduct yourself. This is not considered lying at all. When someone wants to tell you a Torah story or a d'var Torah, and you interrupt them saying "I know, I know, I already heard it," you could be destroying something very precious. Maybe this person just started learning, maybe they needed one student to say "Wow, what a story!" and they would have become motivated to continue teaching. But you destroyed their confidence by saying "I already know." The halacha is clear: even if you've heard the story 50 times, even if you wrote a book about it, you say "No, I don't know, please tell me." This is not lying - this is protecting the honor of the person who wants to teach, encouraging them to continue learning and teaching, and it's also helping them in their own growth because when they teach, they're also learning. The Mishberei Yom teaches us that a truly learned person expresses appreciation by saying "I never heard this before. Thank you for introducing me to this new idea." Never interrupt someone's story, even if you know it. You're not only being respectful, but you might actually learn something new from the same story you think you already know completely.