This is a bug, a difficult to solve bug. The server don't canonize the url, so you have: "http" vs "https", "example.com" vs "www.example.com", "example.com/whatever" vs "example.com/whatever/", "blogspot.com" vs blogspot.co.uk" vs ... , ...
The problem is that canonization is very difficult, it's very easy to get false positives, and each site needs a slightly different canonization rules. So the current method is to use manual moderation.
And when the server restarts, it forgets the old url's for a while, so there are some possible repetitions.
And in a very few times, resubmissions are good.
Most of the resubmissions are accidental, good faith resubmissions. Someone just copy the url and the browser / link / site internationalization was different.
I think there are not official rules, but the unofficial algorithm is:
* When a submission was recently discussed, and it's resubmited the moderators may kill it.
* If the discussion was a long time ago, the resubmission may be left alive.
* If none of the previous submissions get many comments / points, the resubmission may be left alive.
* If there is an intentional systematic resubmission, the moderators may hellban the user and put the domain in the autokill list and other nasty things.