I cannot add my SSH key to my account because of this: "Someone has already added that key as an access key to a repository."
I have checked all the places but this key is not associated to none of the repositories nor users I know of.
I have tried to SSH bitbucket.org, but is does not say any details:
ssh -T email@example.com
authenticated via ssh key.
You can use git to connect to Bitbucket. Shell access is disabled
There's no easy way to work this out because the keys that belong to other accounts should not be shared, so it's not easy to search.
If you shared the key with someone, you should know who. If you were given the key by someone, then you'll need to ask them.
If you have no luck chasing it down, you can raise a support request with Atlassian, and they can search out the key for you and hence identify the account you shared it with.
If you don't want to spend ages chasing down the share, then you can (and should) create a new key for your account and use that instead.