[ColinWright]

There has always been a duplicate detector. If you submit an identical URL, instead of a submission, you upvote the original version and get shown that page.

[wccrawford]

It doesn't always work, though. Sometimes it will still create a new post, even with an identical URL.

[ColinWright]

I suspect it only works for items in the cache, and anything old enough and not recently referenced might only be on disk and not get checked.