This extension appears to be better made and has more features. It's nice to see discussion on HN too.
It does not work reliably. I clicked on a bunch of links from the HN front page. Reddit check did find that they had been posted to reddit before, but Kiwi did not. However all of those links had only been posted once, and had no discussion. Still it seems strange it would say they had never been posted before.
It was also unable to find youtube videos that had been posted before. Youtube is terrible at unique URLs, and I don't blame it. However Reddit Check is able to find all the different places youtube videos have been posted.
I found a link that had been posted to reddit hundreds of times. It only found 11 results. There was also an option for "fuzzy matches" which included a few more links to the exact same URL, but also links that had nothing to do with it. Reddit Check also has a problem where it only returns the first 25 results.
Clicking on any of the links closes the menu, so you can't open many links in new tabs at once. This is also a problem with Reddit Check.
It does not find http versions of https links. Also a problem with Reddit Check.
Clicking on the "submit to reddit" option opened a submission page, but not with the URL in it.
I tried to look at the code but it was all squished together. It does not appear possible to modify it anyway.
Anyway none of these are dealbreakers. I will be using this extension alongside Reddit Check due to the extra features it has. I am concerned about sending so many requests to reddit every time I open a new tab though.