Those who are regular to site and have higher privileges must have noticed that almost every week we have at least one question regarding Betta fish which is duplicate.
Any idea how to solve this issue?
There is no real solution to this. Normally the problem stems from the fact that people haven't searched for existing questions before posting their own. The information is available for the users to read on How To Ask, in fact, that's the first thing on there but a lot of new users will either not be aware that exists or choose not to read the advice.
Secondly, the ask page will suggest duplicates to you as you type your question. In my experience, I've not found this to be the most accurate tool for finding duplicates but it is another source of information for new users and, unlike the help pages, it is directly displayed to every user asking a question.
Finally, Stack Overflow are currently trying to build a better asking wizard which will hopefully help to cut down the number of duplicates and improve the quality of question generally. I'm not sure if there are plans to roll that out to all Stack Exchange site but that would be a question for MSE (if it hasn't been asked already).
Due to the fact that a Betta is a very common 'first fish' and the fact that the information many shops provide to new fish keepers are completely inadequate, these questions will probably keep coming. It deeply saddens me that this is the case because many Bettas will suffer.
All we can do in the mean time is to keep flagging them as duplicates (even posting in the chatroom to get them closed faster) and if you've got any of your own thoughts about how to improve the asking process on the site, post them over on MSE.