you just described exactly what I've done a year ago [1] I always promised myself to rewrite it for Firefox but was too lazy.
They are mostly the same, and Firefox has a compatibility library that stubs most of the incompatibilities.
>Is it possible to avoid duplicates?
I thought about this as well in the past but never actually found a solution. Maybe someone here knows if there's research or an algorithm to uniquely identify the URL.
Edit: typos
https://chrome.google.com/webstore/detail/hacker-news-lookup...