I fail to see the problem with double accounts and why it is a bannable offense. I mean, who cares if someone has two accounts, does this represent a disruption for the forum or something? No, there could possibly even be someone with two accounts both used to contribute positively to the forum. DAs are only a real issue if they are used to disrupt the forum, and in that case people should be banned for forum disruptions not for being a double account.
You are either very new to internet forums or very naive
Of course they are going to be used for some kind of forum disruption, one account is all you need if you want to contribute positively. Of course, I'm not counting here the cases when someone forgot his user and/or password, then created new one instead of recovering the old one, such things are rarely a problem.