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1. How similar are your site's ? how similar are your competitors sites ?
2. You should only get one listing, unless the site's are broken up to represent completely different products. i.e childrens clothes and military ordanance.
3. You can find out the editors name at the bottom of the category, and give them all the urls of your competitors sites relevant to their paticular ODP category and ask them why is this possible.
stickymail me with your urls, i will nose around, though i would suggest that you are industry specific, or product specific to a degree, as SEO's are specialists in particular area's and i can probably point you in the right direction.
unfortunately the area that you are targetting is far removed from any knowledge that i may have in helping you.
One thing i do suggest, is you have got to be careful not to pee off the editors in the ODP, spamming as in duplicate subs to the same cat and related cats is a very poignent way of doing it. Their are alot of odp editors that come here, and i gather that they hate spam and being pestered by a constant barrage of submissions from individuals with sites with duplicate contents, which have no uniqueness seperating each other.
Sorry i could'nt be off more help on this one.