@david and @codinghorror Thank you very much for your support.
The reason why I would like to check the duplicate titles for each category, not all. Because on our system, all categories are private and each category will match to one group. So the condition for viewing, searching the topic in a category is that he needs to be a member of a group matching the category.
If we check the duplicate topic title and users don’t see the topic that has the same title in the category (on our system, it works as a community). It makes the users confused.
Currently, our temporary solution is to “allow duplicate topic titles” on the setting. But if our website is bigger, allowing the duplicate topic title is not good.