I recently asked a [Question][1] on security.stackexchange.com.The question was related to XSS so in my opinion it belonged to the security website.

There was a comment on the question saying that a similar question exists on SO and my question was a duplicate (I think he gave it a down vote). The guy was right but the question was asked on another website (SO). There may be a case when person asking a question might not have an account with the other website.

Is it a good practice to mark questions duplicate if you have seen a similar question on some other website? 


 [1]: https://security.stackexchange.com/q/32616/21234