If you reference for example a JavaScript library from a CDN it will loaded from the browser's cache if it has already been downloaded. But SharePoint doesn't always run in a Internet facing environment. Then you have to store the scripts on the server.
My problem: Say I have 5 app parts on a page, each of those use jQuery. Since all apps exist on its own domain, will jQuery be downloaded 5 times? How do I prevent this behavior? Ideally the browser should only have to download asset X once.