SalesForce_OIDCRegHandler.cls

public class OIDCRegHandler implements Auth.RegistrationHandler{
  public User createUser(Id portalId, Auth.UserData data){
    List<User> users = [SELECT Id FROM User WHERE FederationIdentifier =:data.identifier];
    if (users.size() == 1) {
      return users[0];
    } else {
      return null;
    }
  }

  public void updateUser(Id userId, Id portalId, Auth.UserData data) {
  }
}