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

• Usually one searches the site (either using the big search box at the top of the page, or using Google with the additional filter site:math.stackexchange.com) to check if something has not been posted before. That being said, it's still possible to miss a dupe, so please don't take dupe marks too hard. – J. M. is a poor mathematician Oct 16 '17 at 3:20
• It is rather unclear what your question is. You have marked this as (feature-request), but you do not say what is actually the feature you are requesting. – Martin Sleziak Oct 16 '17 at 3:57
• In case it is helpful, I have collected links to some search-related posts. Some useful post have been posted since that, like “Maybe answered questions” box on “Ask Question” page is much more useful than “Search Q&A” or Announcing a third-party search engine for Math StackExchange.. And you can find a few more in (search) tag on meta. (You can try frequent tab.) – Martin Sleziak Oct 16 '17 at 3:59
• I will also add link to my own answer where I tried to include specific examples how to search for some stuff: How to search math terms/notations accurately and efficiently? Could some experienced users summarize some tips here? There is also a chatroom dedicated to searching - you can also ask there. (Although I will admitted that not many users visit that room.) – Martin Sleziak Oct 16 '17 at 4:02
• The simplest thing you can do is search on google first. If you do this whenever you ask a question, that should avoid most of the duplicate votes. Even when your questions got closed as duplicate occasionally, that's not a big problem. After all, some users are very familiar with the subject and the site, that they might have different (personal) ways to locate duplicates (for example, I stored some standard questions in real analysis in my profile so that I can provide it when needed to). – user99914 Oct 16 '17 at 5:15
• In case the link isn't noticed in Martin Sleziak's comments, Approach0 is awesome. Just saying. – Simply Beautiful Art Oct 16 '17 at 14:14
• May I also suggest changing the title? As is, the title is a bit disconnected from the post. – Simply Beautiful Art Oct 16 '17 at 19:49
• Check on Google if there are sites answering your question. – TheWanderer Oct 17 '17 at 15:10

*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".