The personal CSS is loaded twice, both in an inline <style> tag together with other CSS, and as <link rel="stylesheet" href="https://de.wikipedia.org/w/load.php?debug=false&lang=de&modules=user%7Cuser.groups&only=styles&skin=vector&user=Schnark&version=CLAHufrP&*">.
This seems to be introduced with the latest ResourceLoader changes.
In T122645, @Nikerabbit wrote:
Since some time when using for example Chrome's element inspector, styles defined in MediaWiki:Common.css are shown twice, see screenshot below. This makes it more difficult to try out things by just disabling rules, because now you have to do it twice.