From our standpoint, there's no way, no way at all, to "GET" ANY site listed. It says so right in the submittal policy. I regret that you did not simply believe that: it would have saved you the trouble of whatever you were doing that you thought would "get" a listing. And it would have saved you the frustration of having your unrealistic expectations not met. I repeat, I regret that you did not read and believe; but I don't know what else we could have done to make the truth available to you.
As to the question. It is legal for "exceptional" sites to have multiple listings -- it is no secret; it says so right in the editors' guidelines, which are available to everyone at
http://dmoz.org/guidelines/ .
It is NOT legal to submit a site to multiple categories -- the SUBMITTAL policy (at
http://dmoz.org/add.html ) says to submit to the ONE most appropriate category.(*See below)
It should be obvious that submitters violating the submittal policy (people "trying" to "get" a listing, in short) are the single biggest problem impacting listings for eligible sites. (The problem with ineligible sites is, of course, the editors, whose duty is to not list them.)
*Footnote: there is a well-known exception: sites with content in multiple languages are allowed (no, ENCOURAGED) to be submitted once to the best category in EACH language. (The application of this to the cases you mention is left as an exercist to the reader.) Also, certain sites (the kind of sites should be obvious by looking at the categories involved) are listed once in Regional and again in some Topical category -- if a site has both universal interest and topical focus, it is not unreasonable to submit it twice.
But, and this is the very very important point, editors expected to use good judgment listing sites multiple times; submitters are expected to not multiply submit sites. There is a difference there. What you see in the ODP is what some editor did (which may not even be what he should have done.) What you are allowed to do is what is in the submittal policy.
That submittal policy would help everyone, editors and submitters, if it were followed more often. It does not help any honest person to not follow it.