Problem installing plugin

I'm trying to install the simply_rich_authenticator plugin with the
following command:

ruby script/plugin install -x http://simply-rich-authenticator.googlecode.com/svn/trunk

The problem seems to stem from the -x option which, as I understand
it, used to tell subversion to mark the plugin folder with the
svn:externals attribute.

After running for a while getting files, it returns the following
error:

vendor\plugins\svn\vendor\plugins\tiny_mce' is already a working copy
for a different URL

Any ideas what might be going on. I'm a bit of a newbie; any help
would be appreciated!

Thanks

Actually, I think part of my problem is that I mistook this for a plug
in when in fact I think it is a complete sample application
demonstrating the use of restful_authentication...