In French there are exist many family names that are indistinguishable from last names, such as, e.g., Édouard Philippe vs. Philippe Édouard. The phenomenon is so widespread that occasionally serves as a basis for humor pieces. There are also cases in English, although perhaps less widespread - the first thing that comes to my mind is Olivia Newton-John (although the double last name is unlikely to cause confusion, this is probably a composite of what was initially two separate family names.)
On the other hand, many languages have markers for distinguishing last names from first names, such as -ov/-ić/-skii in Slavic languages, -oglu in Turkish, ben/bin/ibn in Semitic languages and -sohn in some Germanic ones - the family names originating from first names do not pose any problem in these languages.
Are there any family names in modern German that are confusable for first names? If yes, how widespread is this phenomenon? If not, what are the typical features permitting to distinguish first names from family names originating from first names?