"Jeremy is correct when he says that multiple users cannot share email addresses or user names. It's impossible because our database forces those fields to be unique.

What's happened to you is that you've accepted multiple invitations (3 of them - 2/8, 2/9, and 3/5) with the same account. That's fine. It doesn't mean that you have multiple accounts. The invitations list merely shows you which email address the invitation was sent to and which account accepted it. If you're using the same computer for multiple accounts, make sure you Sign Out before accepting an invitation.

Now, to your original problem - What notifications aren't you receiving?"