Agreed! I got those flags as moderator and couldn't figure out what they were referring to. Ideally a Duplicate Flag should require an actual target. But short of that (which would be complicated because the additional information for each type of flag would be different), a text field to fill in details (in this case, duplicate target) would make a huge difference.

This is a bug. The duplicate close reason is supposed to require a link. I just checked the configuration here on meta and it's set to yes (required), but I was able to flag this as a dupe without supplying one. Thanks for the report.

Oh wait, I see what's going on -- if you close as a duplicate you have to supply a link, but if you flag you don't. That's an oversight. I don't know what it'll take to fix this. At the very least, we could add some text asking people to supply the link in the textbox.

@MonicaCellio i think there should a easy way to do it. if user selects duplicate than the text field can't be empty. Like it does when we select other. I will try to check the code(I am not a Ruby programmer that's why i always don't understand those codes). After looking at the source, I noticed community is taking Is a duplicate flag from database. And, It doesn't have for answer.