WIP: Show TBD message if username is duplicated

lib/models/external-account.js

@@ -60,6 +60,8 @@ class ExternalAccount {
         else {
           debug(`ExternalAccount '${accountId}' is not found, it is going to be registered.`);
 
+          // TODO count and throw error if username is dupricated
+
           return createUserFunction().then((user) => {
               return this.create({ providerType: 'ldap', accountId, user: user._id });
             });