That is doable, but quite an amount of work and requires more parsing of SpamCop-generated pages. So it would make the module more dependent on the layout and content of the page etc. and more vulnerable to any changes that SpamCop would make to the page.

So let's ask ourselves, is it really useful enough to justify the changes?

The duplicates can be of 2 types AFAIK:

1) The exact same message ( many copies of one instance of the message - all having identical body and headers, especially Message ID) - removing such duplicates makes sense, but they occur only if you accidentally submit one message many times.

2) Many instances of the same message (to be exact, many messages with the same body, but differing with their Message IDs) - they actually should be reported as separate spams, even if they come from the same ISP, to allow for more accurate spam traffic statistics for this ISP.

It seems a good idea to implement this functionality for problem 1), but it will be considerable amount of work. Problem 2) isn't a problem, actually.