1. Proposed dupe has at least one score 0 answer
2. Vote up said answer
3. VTC
4. Revert vote

Besides that, I would seriously question the idea that disallowing dupes to unanswered questions will help increase dupes to answered questions.  Are we hoping that people will search (and find) answered dupes after being unhelpfully foiled?  Or is there a problem with users voluntarily choosing unanswered questions over answered ones in their choice of dupe?