How does this affect the SEO of the website and does it also affect
other websites that are hosted on the same server?
Just the generation of a subdomain wouldn't affect the website. A cPanel add-on domain is unlikely to have an effect on another domain which are in general completely scoped from each other.
When it has an effect, will that effect be positive or negative and is
there a way to decrease the effect?
When you use hyperlinks from the website to the subdomain it can potentially have a negative effect when the exact same content is served. When you don't point out or use the created subdomain (using hyperlinks), it will be unlikely to have an effect on search engine optimization.
Does Google treat the content on the subdomain as duplicate content?
The duplicate content subject is kind of complicated, when it comes to Google. As far as I concern Google sees duplicated content when it is duplicate content. As simple as that. When the exact same website is served on a subdomain, Google (and other search engines) can potentially see it as duplicate content when it gets indexed.
You can just avoid that from happening or choose a preferred domain in Google's webmaster tools.