I've deployed a fix for people affected by sign-in issues caused by having multiple accounts associated with the same email address. Here's how to activate the fix:
You must attempt to sign in using the email address associated with your account (not a username) and must have access to that mailbox.
If there are multiple accounts associated with the email address then you will be shown a warning message with the option to have an email sent to repair your account.
The email will contain a link to a page showing all the accounts associated with the same email address and allow you to select the one you want to be active (it will disassociate all the other accounts from the email address).
Once you have completed the process then you should be able to sign in as normal. If this has solved your issue then please reply on this topic to let me know it worked for you.
If you still can't sign in then remember to give the password reset process a try first (even if you already tried it before and never received the email).
If your sign-in issue isn't as described above then please email me even if you have already done so before: Keeping track of all the emails and who's issues have been fixed is difficult so it's easier to address the most common problems and whittle down the list.
I'm hoping that the only people who may still have a problem are those who may have multiple accounts associated with the same email but no longer have access to that email address.