[GREEN] Implement handler for existing user on account creation

apps/accounts/views.py

@@ -51,13 +51,27 @@ class AccountViewSet(viewsets.ModelViewSet):
             return AccountRegisterSerializer
         return AccountSerializer
 
-    def perform_create(self, serializer):
+    def create(self, request):
+        serializer_class = self.get_serializer_class()
+        serializer = serializer_class(data=request.data)
+        serializer.is_valid(raise_exception=True)
+
         username = serializer.validated_data.pop("username").lower()
         password = serializer.validated_data.pop("password")
 
+        if User.objects.filter(username=username).exists():
+            return Response(
+                {"username" : "User with that username already exists"},
+                status=status.HTTP_409_CONFLICT
+            )
+
         user = User.objects.create_user(username=username, password=password)
         Account.objects.create(user=user, **serializer.validated_data)
 
+        return Response(
+            serializer.data, status=status.HTTP_201_CREATED,
+        )
+
     def perform_destroy(self, instance):
         instance.delete(author=self.request.user.account)