added comments and fixed logic issue

backend/src/main/java/ovh/herisson/Clyde/Services/ScientificPublications/ResearchesService.java

@@ -110,6 +110,8 @@ public class ResearchesService {
     }
 
     public Researcher saveResearcher(Researcher researcher) {
+
+        if (researcherRepo.findByUser(researcher.getUser()) != null) return null;
        return researcherRepo.save(researcher);
     }