Most of the time, you don't want to see unanswered questions in the duplicate dialog, since you can't normally close as a duplicate of unanswered questions. This behavior is intended, at least as it says here:

The search results include only answered questions (per the usual SO definition of "answered", where the question must have an accepted or up-voted answer).

But there are times when you can close against something unanswered:

• On meta
• Questions with the same author

But the dialog doesn't show unanswered posts in those circumstances.

In the past, I have wondered why I was mysteriously unable to find some specific posts in the duplicate dialog. For example:

• I saw two near-identical posts by the same user. I was unable to find the duplicate in the dialog, even after typing in the exact tile or using the user:1 search trick.

• On meta, this post of mine does not show up in the dialog. I posted about this before, on MSO, but at that point I only knew it affected that one post (the joke's on whomever got rid of "too localized" I guess).

In both cases, I had to go get the URL and paste it in manually.

The best way to see this is to use answers:0. I hope you don't think there are only two unanswered questions here!

Most of the time, the results for this search are merged questions (but not all the merged questions). I have no clue why that one post shows up here. (Maybe it was merged and un-merged at some point?) This current behavior is pretty useless.

It should be very easy to fix the dialog on meta. It might take a little more effort to have the same user's posts show up on main, but I can't imagine it being prohibitively difficult.