Redmine

Redmine can register plugin same ID with registered one.
Currently, below problems exist.

• @registered_plugins[id] is overwritten.
• /admin/plugins shows only one plugin
• but load_path, view_path, ... are changed

I want to reject and log warning second or later plugins.

functional/settings_controller_test.rb was already registered foo plugin twice. I cleared on teardown to pass unit test.

This ticket related to #27705.