我特雷 gitlab-ee-11.8 ；gitlab-ce-11.8
新用户可以登录并创建从人群到 gitlab 的帐户；但存在用户不能。
登录失败，因为电子邮件已被占用
https://docs.gitlab.com/ee/integration/omniauth.html#enable-omniauth-for-an-existing-user
为现有用户启用 OmniAuth
现有用户可以在创建帐户后为特定提供商启用 OmniAuth。例如，如果用户最初使用 LDAP 登录，则可以启用 OmniAuth 提供程序，例如 Twitter。按照以下步骤为现有用户启用 OmniAuth 提供程序。
Sign in normally - whether standard sign in, LDAP, or another OmniAuth provider.
Go to profile settings (the silhouette icon in the top right corner).
Select the “Account” tab.
***Under “Connected Accounts” select the desired OmniAuth provider, such as Twitter.** where ? I can't find!!!*
The user will be redirected to the provider. Once the user authorized GitLab they will be redirected back to GitLab.