Posting a question on this thread hoping someone will know. I tried calling the number on the census form to ask them and couldn't manage to get a human so I gave up and hung up.
Prompted by a combination of a TV advertisement, boredom, and civic duty, I logged on to the census website to fill it out. I had not received my letter with my code, so I chose the option to compete the census without it.
Since then, I've received my letter with my code in the mail. I don't want to fill the census out twice and be double counted, but I also would like to stop getting the mail about it as I feel like I am being pestered and there is an implied accusation that I'm derelict in my civic duty.
At the moment I'm deciding to toss the letters with codes in the trash, and explaining the situation if census workers do happen to show up, and not worrying about it. If anyone has any better advice what to do, I'd be interested in hearing it, especially if there's some option to say on the website "Here's my code but I already gave you my answers earlier"