Is it a good policy to delete our question, when we found our question is duplicate of another question? or another policy which you suggest ?
Leave it there. It serves to channel people with the same problem in the right direction.
If the target duplicated uses exactly the same keywords as another, i.e., it should have turned up the original question in a search, we can just delete it. It doesn't help SEO to have the same content with little to no variation whatsoever.
I have found useful additional information in duplicate questions. Things such as flags set or solutions tried that affect my approach. If those questions are deleted, then that useful information disappears.
Is it possible to have a "Related" categorisation that might be reviewed as the information becomes too old to be useful? For example, when a suggested solution uses a deprecated function, or applies to an old kernel.
I think if the questions and answers are an exact duplicate, then delete the older / less precise / less accurate thread. However, not all support related scenarios are identical. Theoretically every question and answer may potentially help person A but not person B and vise versa.