In this case the URLs differed by a trailing slash. Normally I do an auxiliary search to verify whether someone already submitted something, but I didn't have time yesterday.

I'm not sure I agree with the GP that the occasional duplicate is harmful. It may be that a small amount of randomness in these things contributes to the life of the site.

I don't really think occasional duplicates are harmful as such - it is the way many submissions are just never really noticed, like the original submission of this one, that I think tend to depress submitters.