Prevent redeclaration of LDAP strategy

config/initializers/7_omniauth.rb

 if Gitlab::LDAP::Config.enabled?
   module OmniAuth::Strategies
     server = Gitlab.config.ldap.servers.values.first
-    const_set(server['provider_class'], Class.new(LDAP))
+    klass = server['provider_class']
+    const_set(klass, Class.new(LDAP)) unless klass == 'LDAP'
   end
 
   OmniauthCallbacksController.class_eval do