If you are seeing multiple calls to fs.js, it's very likely that you have several iframes on the page in question.
The FullStory data capture script is architected so that each iframe loads its own version of fs.js. If a page contains two iframes, you will see three total fs.js requests (one for the outer page and one for each iframe).
Not to worry! The redundant calls are to the initially loaded script which should be cached (unless the user has disabled caching within the browser). So, in terms of client performance there should be no impact as all requests beyond the first should be served from the client's browser cache.