This may have been suggested before

Seems like a major problem is caused by submitters making multiple submissions of the same site or deep links. Creates a lot of work for you guys when you have to clean that garbage out before you can even study a site.
Is it not possible to have the submission page check to see is the same domain name is listed on dmoz or in the “waiting for review” section. FFA links pages do this all the time to reduce this type of spam.

Yes, been suggested before. Yes, I'd really REALLY like to have something like that.

Unfortunately, it's more difficult than it looks. Consider domains like Tripod that have many many listings -- some as subdomains and some as subdirectories; you have domains like the BBC that ought to be massively deeplinked much more than they are. And you have spammers using multiple subdomains to look like multiple sites to the search engines.

Another consideration: we editors do not want to tell submitters everything we know about a domain. Such information allows us to trip up stupid (or careless) spammers much more easily than if they knew what we know, and were successfully crafting their submittals to look like unique ones. It would add about 20-30% to our workload if spammers were made immediately aware of everything editors can easily know about them -- and that, in turn, would add more than a few months to the average wait time for review. So such enhancements are not likely to show up on the public submittal pages.

It could be possible to check to see if a URL is already listed from the RDF dump and provide that as status - but then the submitter can just as easily do a search and find that information out themselves. We don't want to eliminate completely the possibility of submitting an already listed site, since many can be legitimately listed in both topical, regional, and/or various World languages.

It isn't easy at all to do a check of what is waiting in unreviewed somewhere in the directory. Each and every category has its own little file, and it is impossible for the server to load all of those files and then examine the contents. A submission of the same url to the same category simply overwrites the previous submission. Editors have internally developed tools to help manage the unreviewed so it isn't too hard to delete a lot of duplicates, but it does take time away from more productive editing.

Thanks for the suggestion!