drupal_add_js only once
It depends what type of javascript you are adding. 


drupal_add_js will only add files once, no matter how often it's called ;
Settings would overwrite previous values, so it's not a major problem ;
Inline code would indeed get added multiple times.


Given that your script needs including only once no matter how often the field is present, I assume you are not generating it's code on the fly - so putting it in a file would be an option.

Otherwise, as suggested by Clive, you could have your own wrapper around drupal_add_js that ensures a given inline js is only added once by using a static variable.