A question was asked in one of the idea discussions about how to link to duplicates. Go to the page for the original idea and copy the URL from the browser's address bar. Go to the duplicate idea and add a comment. In the comment editor there is a button that looks like an "A" with a chain link next to it. Click on this and paste the URL from the other idea into the form that appears. Do a quick sanity check on the URL. I've been getting some accidental duplicate "https:" at the beginning of the link.

I think this is similar to linking to other forum posts, but I don't do that often enough to remember.