This is not possible for a normal user
You can't. The dialog would tell you that you can't mark the question as a duplicate if the other one doesn't have an answer.
Go on the unanswered tab and select any question. It can even have answers, but if they are in that tab those answers don't have any upvotes (yet).
Copy the link.
Go to the active tab.
Choose any question that is not currently on hold.
Click "Close" and choose "Duplicate".
Enter the link you just copied.
You get a red text that reads:
This question does not have an upvoted or accepted answer
(Important step: close the dialog as this was just a test)
Diamond mods are the only ones who could do this
They probably shouldn't in most cases. I don't see any possible reason except for maybe a new user spamming the exact same question. Maybe because someone commented that they want to close it and the new user thought "Well, let's try again". But that's about it and I think the system would prevent them from asking the exact same question again. If they just change a word or two the system might not detect it and the mod should close as a duplicate, but normal users can't do that. Though the normal process would probably be to merge the questions as @Michael pointed out in the comments.
I don't really see a situation where this mod power is useful. Either there are answers on the other question and it would be useful to close as a duplicate, which would allow normal users to do this, or it's the exact same question to the letter, which would necessitate a merge. But theoretically mods could potentially do this.