https://www.cnrtl.fr/definition/concoursLook at older meanings of "concours", as legalese tends to keep alive archaeological layers of language.
It's possible to presume a meaning that would make sense, but that can be dangerous business, leading to some apparently correct translation that is anything but.
What is more or less sure is that you have to look for the whole of "une réparation d'un préjudice aux concours propres", and first find out if "aux concours propres" relates to "une réparation" or to "un préjudice".
The few ghits about "aux concours propres" are all about exams, no help of any kind there.
Also, can you give a reference to this case? This decision of "La Cour de cassation" MUST have been published, if nowhere else on
www.legifrance.gouv.fr.