Per your comment, I wrote a Chrome extension that creates links to other HN submissions pointing to the same URL. Hovering over the links provides additional details.

This provides similar functionality as the 'past' link in a submission's subtitle, but shows the duplicates automatically.

Here's the extension:

https://chrome.google.com/webstore/detail/hacker-news-duplic...

Here's the source code:

https://github.com/dstein64/hn-duplicate-detector