Having done that at least once on the mother meta, and and on SU. Its tricky but doable
In your specific cases I'd consider another option though -
Turn the question into a problem statement - "hey, so it happens that people flag stuff as a dupe of something that happens somewhere else, and we don't know where it is - it would be nice if folks supplied the link. That said, what does the community think, and do we have any other approaches to solve the problem?
Sometimes you might even get a better answer you didn't really want but is .
So lay out a problem definition - state that "this might work" and throw on a discussion flag and see what people discuss.