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.