Let them be there, if they aren't breaking up your rules(except double registratin) Sometimes, this could be result of dynamic IP.
Yes. I see what you are saying about the commercial posting. Never thought of that. I know on DP the signatures are the same for all your posts. Say if you put link1 in your signature 2 weeks ago, and then put link2 in your signature yesterday, all the posts you have made to date would change to the same signature that you just put in with link2. So they could be double posting under different names so they can have back links to different sites. Or if you have a signature link limit, then they could be creating the 2 accounts to add more to their link limit to promote products with.