email already taken

my new email address eBay say's is already in use, how do I find out how that happened ?

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Well that is quite likely, I did sign in a few times with Facebook, but yesterday I changed my Password and I logged in using that new PW, then attempted to change the eMail and that's when it said eMail already taken, so I am at a loss as to how to get rid of my old one then add the newer one...