Update SAML first year attach logic

src/main/java/nl/tudelft/ewi/queue/service/SAMLUserDetailsServiceImpl.java

@@ -63,7 +63,7 @@ public class SAMLUserDetailsServiceImpl implements SAMLUserDetailsService {
 
 	private void AttachUserToFirstYearStudent(User user) {
         FirstYearStudent firstYearStudent = firstYearStudentRepository.findByNetId(user.getUsername());
-        if (firstYearStudent != null) {
+        if (firstYearStudent != null && firstYearStudent.getUser() == null) {
             firstYearStudent.setUser(user);
             firstYearStudentRepository.save(firstYearStudent);
         }