I have recently decided to change the email address that my Gravatar is associated with. In the end, I had to create a new Gravatar account with the different email address. After doing this, I successfully changed my email address for my StackOverflow, ServerFault, and AskUbuntu accounts. However, when I attempt to change my SuperUser email address, I get an error saying "Oops! ... Email is already registered". If the email worked in the other accounts, why is it not working here?
It looks like you've accidentally created two accounts on SU.
The one you've used to post this question - 51597 - is linked to accounts on the other sites and so is presumably your "main" account.
But there is another account (51596) which has your name and matches your gravatar (implying it is using your email address) of the accounts linked to the first.
You need to get the two accounts merged, I believe the simplest way to do that is to flag one of your own posts of moderator attention asking for a merge and providing both your user IDs.
I have merged the two account DMA found into one here: