Hmm, hard to say. SuperSlider-Excerpt adds the excerpts using the "get_the_excerpt" filter, which Relevanssi calls once per excerpt, as it should. So, I don't know where the second one comes from.
I took a look at the plugin and well... First of all, it does produce a rather large number of notices and errors. However, looks like if take a look at the superslider-excerpt.php file and find this at line 308 or so:
global $wp_query, $wpdb;
$id = $wp_query->post->ID;
$post_title = $wp_query->post->post_title;
and change it to this:
global $wp_query, $wpdb, $post;
$id = $post->ID;
$post_title = $post->post_title;
it might work a bit better. It still doesn't work very well, though, and like I said, if you enable debugging, you'll notice the plugin spits out a ton of notices and warnings. SuperSlider-Excerpt hasn't been updated in more than two years, and it's starting to show.
If all you want is a thumbnail in search results, there are other - perhaps easier - ways to achieve that.