If you see the error "We encountered a problem adding your identity to CircleCI... The identity provided by your VCS service is already associated with a different account. Please try logging in using that account or contact us for further help."
This means that the account has previously been authorized with CircleCI.
This can be solved by revoking CircleCI's OAuth token within your VCS account and attempting to authorize it once again.
If you still are unable to connect your VCS account, please raise a ticket with our support team and we will manually disconnect your VCS account for you.