Warn and return for duplicated plugin

Warn and return for duplicated plugin

If a plugin has been loaded already, we should warn and return, instead
of adding it on the plugin list.

	PR ld/20321
	* plugin.c (plugin_opt_plugin): Warn and return if plugin has
	been loaded already.
	* testsuite/ld-plugin/lto.exp: Run PR ld/20321 test.
	* testsuite/ld-plugin/pr20321.c: New file.