Interesting, do you know of any specific examples?
I'm seeing 408 bans for ban evasion/dupe accounts from the period covering 2004/2012

It's safe to say an awful lot of people reregister!
But in terms of dupe accounts...
We don't have enough manpower to cover all the actual threads that need moderation, let alone worrying about that stuff. 99% of the time when someone gets banned for having an alt account, it's one of three things:
- Someone is banned and immediately starts using an alt account they had previously registered, doing the exact same thing that got them banned.
- Someone is permed and re-registers with an account that is so clearly an alt they get picked off shortly after account approval (BannedUser -> BannedUser2 or NotBannedUser or some other clever variant of their original username)
- Someone successfully reregisters and uses the site, and during a temp ban for some random issue, it becomes obvious from checking the user's account that they're a ban evasion.
We don't go out of our way to worry about people being banned in 2004 or 2006 or 2007 and re-registering years later as successful posters. Shit, EviLore was what, 16 when he took over GAF, and he's in his late 20s now. Tons and tons of people we've permed are genuinely different, better posters now in future incarnations. It's just not something worth spending effort investigating too closely.
That being said, the Terms of Service says:
n. Each accepted member is granted one, and only one, username. If you are caught using multiple usernames, you will be subject to immediate and permanent banning of all accounts involved.
So duping or ban evasion will always be bannable, obviously.
All the most eccentric, hilarious, and entertaining people get banned eventually. Its a shame really.
Function of the forum getting huge. much easier to have joke characters and persistent trolls when the forum gets 100 replies a day and the regulars, who all know each other, can laugh about it than it is when the forum gets 10,000+ replies a day and many of the members don't know or appreciate someone's schtick and it derails or interferes with other conversation. I think you'd find this with most forums that get larger.