I got this error
Someone has already added that key as an access key to a repository.
when trying to insert my public ssh key whereas no repositories of mine have my ssh-key inside access key menu
There's no easy way to work this out because the keys that belong to other accounts should not be shared.
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.