I have found a question on SO, which is very similar to a question on the SE community (https://drupal.stackexchange.com/).
I have tried to create a flag with the standard flags for it, but it seems that SE community isn't allowed. I am getting the following message:
The duplicate question must exist on Stack Overflow
So I wrote a custom comment to point to the duplicate similar to the following message:
Possible duplicate of URL
This was declined with the comment:
Please use standard close votes and flags for this.
What is the recommended way to show up duplicates on SE sites?