This was not duplicate data. I run Census+ on two accounts, because I have characters on both Alliance and Horde on a PVP server (omg don't tell Blizz!). When I've been submitting data, I typically submit the Alliance data first, then immediately submit the Horde data. I only just noticed the above message, but my guess is I've lost legitimate Alliance data submissions over the past couple months because of this.
Maybe add the source path or something to the cookie you use to track recent submissions? In any case, it'd be awesome to be able to submit from multiple accounts without having to wait an hour between
