I don't know if here is a right room but i ask my question here anyway. Is it possible if a user aks a popular question and answer it by himself ? For example, questions like ' what is the difference between engineering and true stress' have been asked many times and we still get similar questions frequently. Is there anyway to redirect the users before they post their questions to the answer that's already granted by moderators?

The "mark as duplicate" close vote system is the main solution to this problem. There's no algorithm which can understand the question to be asked and comparing it to others which already exist. You'll see that when you write a question, there's the "Questions that may already have your answer" list, but that's basically just doing similar-word comparisons. But many questions can be asked with similar words, so it can't determine whether you're actually asking the same question.

This Jeff Atwood blog post on duplicates is quite relevant. Basically, duplicate questions are actually useful since they improve the site's "findability" for users Googling for answers by covering a greater variety of possible search terms for the same problem. While mods can merge questions which are absurdly similar, that's rarely done (don't even know if it's ever been done on Engineering.SE).

As to your particular example, I can really only find two "true vs engineering stress" questions, and they're pretty different. One is about why we use engineering stress instead of true stress (with little discussion of how they're actually different). The other is three hours old and is about how they're actually different.

@SamFarjamirad (just to notify you)