Submitted many, many times:

http://searchyc.com/pybrain?sort=by_date

Some of these have substantial commentary.

The last one with an exact URL match was ~1 year ago. On the other hand, I just tried submitting a URL that someone else submitted 5 minutes, previous and HN translated that into an up-vote for the pre-existing submission.

That's the way the duplicate detection works here on HN. A recent exact match becomes an upvote, because if you think it's worth submitting then you would obviously think it's worth upvoting the original.

On the other hand, since old discussions are closed, repeats are allowed after a certain time. Likewise, if the URL isn't an exact match, it's not recognised.

What is the time delay limit?

However long it takes for the story to be removed from memory. HN only does dupe detection on the stories that are already in memory.