I commented out this check, because the way the build process works is that the CKEDITOR.skins.icons[] array pretty much becomes immutable for those plugins built into ckeditor by ckbuilder.jar.  A call to this this method should always expect to override any value that is already set.  This does bring up one issue where "non-built-in" plugins that are included via ".extraPlugins:" are currently running AFTER skin.js from a custom skin, which means the plugin overrides any value set by skin.js.  I'm going to implement a separate commit for that, as skin.js should always run "last" so that that it skin's skin.js becomes definitive.