diff --git a/src/verzoeken/api/serializers.py b/src/verzoeken/api/serializers.py
index 3410679..9dbdb0f 100644
--- a/src/verzoeken/api/serializers.py
+++ b/src/verzoeken/api/serializers.py
@@ -32,7 +32,10 @@
 )
 from verzoeken.sync.signals import SyncError
 
-from .validators import ObjectVerzoekCreateValidator
+from .validators import (
+    KlantProductUniqueTogetherValidator,
+    ObjectVerzoekCreateValidator,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -227,6 +230,16 @@ class Meta:
                 ],
             },
         }
+        validators = [
+            KlantProductUniqueTogetherValidator(
+                queryset=VerzoekProduct.objects.all(),
+                fields=["product", "verzoek"],
+            ),
+            KlantProductUniqueTogetherValidator(
+                queryset=VerzoekProduct.objects.all(),
+                fields=["product_code", "verzoek"],
+            ),
+        ]
 
     def validate(self, attrs):
         validated_attrs = super().validate(attrs)
diff --git a/src/verzoeken/api/tests/test_verzoekproduct.py b/src/verzoeken/api/tests/test_verzoekproduct.py
index bc3555c..117a8e5 100644
--- a/src/verzoeken/api/tests/test_verzoekproduct.py
+++ b/src/verzoeken/api/tests/test_verzoekproduct.py
@@ -114,6 +114,22 @@ def test_create_verzoekproduct_with_invalid_product_url(self):
         error = get_validation_errors(response, "product")
         self.assertEqual(error["code"], "bad-url")
 
+    def test_create_verzoekproduct_with_product_url_not_unique(self):
+        verzoek = VerzoekFactory.create()
+        verzoek_url = reverse(verzoek)
+
+        VerzoekProductFactory.create(verzoek=verzoek, product="https://example.com/")
+
+        list_url = reverse(VerzoekProduct)
+        data = {"verzoek": verzoek_url, "product": "https://example.com/"}
+
+        response = self.client.post(list_url, data)
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+
+        error = get_validation_errors(response, "nonFieldErrors")
+        self.assertEqual(error["code"], "unique")
+
     def test_create_verzoekproduct_with_product_id(self):
         verzoek = VerzoekFactory.create()
         verzoek_url = reverse(verzoek)
@@ -133,6 +149,22 @@ def test_create_verzoekproduct_with_product_id(self):
             verzoekproduct.product_code, data["productIdentificatie"]["code"]
         )
 
+    def test_create_verzoekproduct_with_product_id_not_unique(self):
+        verzoek = VerzoekFactory.create()
+        verzoek_url = reverse(verzoek)
+
+        VerzoekProductFactory.create(verzoek=verzoek, product_code="test")
+
+        list_url = reverse(VerzoekProduct)
+        data = {"verzoek": verzoek_url, "productIdentificatie": {"code": "test"}}
+
+        response = self.client.post(list_url, data)
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+
+        error = get_validation_errors(response, "nonFieldErrors")
+        self.assertEqual(error["code"], "unique")
+
     def test_create_verzoekproduct_without_product(self):
         verzoek = VerzoekFactory.create()
         verzoek_url = reverse(verzoek)
diff --git a/src/verzoeken/api/validators.py b/src/verzoeken/api/validators.py
index 70deb67..a231cc2 100644
--- a/src/verzoeken/api/validators.py
+++ b/src/verzoeken/api/validators.py
@@ -5,6 +5,7 @@
 from django.utils.translation import ugettext_lazy as _
 
 from rest_framework import exceptions, serializers
+from rest_framework.validators import UniqueTogetherValidator
 from vng_api_common.models import APICredential
 from vng_api_common.validators import ResourceValidator
 from zds_client import ClientError
@@ -106,3 +107,18 @@ def __call__(self, attrs: OrderedDict):
             raise serializers.ValidationError(
                 self.message.format(object=object_type), code=self.code
             )
+
+
+class KlantProductUniqueTogetherValidator(UniqueTogetherValidator):
+    def enforce_required_fields(self, attrs):
+        # These attributes are optional, so this should not be enforced
+        return
+
+    def __call__(self, attrs):
+        # To ensure that the validation does not fail in case the attributes
+        # are missing (since they are not required)
+        if ("product" in self.fields and "product" not in attrs) or (
+            "product_code" in self.fields and "product_code" not in attrs
+        ):
+            return
+        super().__call__(attrs)