For a post with a manually created excerpt the more link is doubled.
To modify the more link of manually created excerpts hook 'get_the_excerpt' has to be used. This seems to conflict with the behaviour of this plugin which calls the function get_the_excerpt() by it's own and does not handle the more link properly. Thus the more link is doubled when get_the_excerpt() is called a second time in template content.php in theme TwentyEleven.
This does not occur with automatically created excerpts which may be customized using hook 'excerpt_more'.