If you try to leave a comment and get an error saying “That email address is associated with an existing WordPress.com account, please log in to use it”, that’s because WordPress (the blog hosting software and platform) recently updated their comment system, and they changed something. A cursory glance online reveals many sites across the blogosphere are having this issue.
So if you had a problem leaving a comment, please try again. If you do have an account, you should be able to login and leave a comment. I don’t have any way to see the info from who logs in, so don’t worry about that.
I’ve discovered this also happens for e-mail addresses that have a Gravatar account (which is for the image that goes with your name). That seems to be a bug, as far as I know. So in the meantime, you can either use an alternate e-mail address (or make one up), or just wait until it’s fixed.
Sorry for the inconvenience.