Is there a way to verified if an add_filter is already applied?

I have in my theme this filter:

add_filter('widget_text', 'do_shortcode');

Now I’m developing a plugin where I have that same filter. I would like to know if is there a conditional that I can check if filter is already placed? I don’t want to run it twice. I don’t know if that may cause any problem. I just want to play safe.

<?php has_filter( $tag, $function_to_check ); ?> 

http://codex.wordpress.org/Function_Reference/has_filter