Many times when I ask a question I found that it blocked as it is duplicate. But I don't know which question is ask before. So how can I understand that the question is duplicate or not?? Please help me
Because searching for fomulas with symbols in them is hard*, even if you try to find duplicates you may fail. Then go ahead and post. If it is closed as a duplicate, you lose no points, and the linked question gives you your answer.
If you are asking to find the answer (as you should) and not merely to gain reputation, this is exactly what you want.
*The AI in the search engines do not yet know that $xy=yx$ matches $ab=ba$. Even less do they know that both of those match "commutative law".