Index: backend/auth_form/serializers.py
===================================================================
--- backend/auth_form/serializers.py	(revision b49d770d5f27fe930c5f8c9dd33d16e363b7be64)
+++ backend/auth_form/serializers.py	(revision aba5377d31dbac4f86f796be4f3e4b7f569ed854)
@@ -78,5 +78,4 @@
         rep = super().to_representation(instance)
         rep['passed_subjects'] = SubjectSerializer(instance.passed_subjects.all(), many=True).data
-        # Fix: fetch Subject objects for each semester's subject IDs
         rep['passed_subjects_per_semester'] = {
             str(sem): SubjectSerializer(
Index: backend/auth_form/views.py
===================================================================
--- backend/auth_form/views.py	(revision b49d770d5f27fe930c5f8c9dd33d16e363b7be64)
+++ backend/auth_form/views.py	(revision aba5377d31dbac4f86f796be4f3e4b7f569ed854)
@@ -1,2 +1,3 @@
+from .models import Student
 from .serializers import RegistrationSerializer, LoginSerializer, StudentFormSerializer
 from rest_framework import serializers, status, views
@@ -64,8 +65,6 @@
     
     def post(self, request):
-        print(request.data)
         if hasattr(request.user, 'student') and request.user.student.has_filled_form:
             return Response({"detail": "Student profile already exists."}, status=status.HTTP_400_BAD_REQUEST)
-        
         serializer = StudentFormSerializer(instance=request.user.student, data=request.data)
         if serializer.is_valid():
@@ -74,4 +73,9 @@
             request.user.student.save()
             return Response(serializer.data, status=status.HTTP_201_CREATED)
+        
+        index_match = Student.objects.filter(index=request.data['index'])
+        if index_match.exists():
+           return Response({'message': "Постои студент со тој индекс."}, status=status.HTTP_400_BAD_REQUEST)
+
         return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
     
@@ -80,4 +84,9 @@
             return Response({"detail": "No student profile found."}, status=status.HTTP_404_NOT_FOUND)
         serializer = StudentFormSerializer(instance=request.user.student, data=request.data)
+
+        index = request.data['index']
+        index_match = Student.objects.filter(index=index).exclude(pk=request.user.student.pk)
+        if index_match.exists():
+            return Response({'message': "Постои студент со тој индекс."}, status=status.HTTP_400_BAD_REQUEST)
         if serializer.is_valid():
             serializer.save()