diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index 3fcc85c3b..7883bd80d 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -947,6 +947,11 @@ class ModelSerializer(Serializer):
         # Determine any extra field arguments and hidden fields that
         # should be included
         extra_kwargs = self.get_extra_kwargs()
+        for field in extra_kwargs:
+            assert field not in declared_fields, (
+                "Field {} is declared on the class and also present in "
+                "extra_kwargs or read_only_fields".format(field)
+            )
         extra_kwargs, hidden_fields = self.get_uniqueness_extra_kwargs(
             field_names, declared_fields, extra_kwargs
         )
@@ -1321,12 +1326,11 @@ class ModelSerializer(Serializer):
                 # add in a hidden field that populates it.
                 hidden_fields[unique_constraint_name] = HiddenField(default=default)
 
-        # Update `extra_kwargs` with any new options.
+        # Update `extra_kwargs` with any new options but don't overwrite old values.
         for key, value in uniqueness_extra_kwargs.items():
             if key in extra_kwargs:
-                extra_kwargs[key].update(value)
-            else:
-                extra_kwargs[key] = value
+                value.update(extra_kwargs[key])
+            extra_kwargs[key] = value
 
         return extra_kwargs, hidden_fields
 
diff --git a/tests/test_model_serializer.py b/tests/test_model_serializer.py
index 096cbc8d6..b812a6791 100644
--- a/tests/test_model_serializer.py
+++ b/tests/test_model_serializer.py
@@ -933,8 +933,8 @@ class TestMetaInheritance(TestCase):
 
             class Meta:
                 model = OneFieldModel
-                read_only_fields = ('char_field', 'non_model_field')
-                fields = read_only_fields
+                read_only_fields = ('char_field',)
+                fields = read_only_fields + ('non_model_field', )
                 extra_kwargs = {}
 
         class ChildSerializer(TestSerializer):
@@ -955,3 +955,44 @@ class TestMetaInheritance(TestCase):
         self.assertEqual(unicode_repr(ChildSerializer()), child_expected)
         self.assertEqual(unicode_repr(TestSerializer()), test_expected)
         self.assertEqual(unicode_repr(ChildSerializer()), child_expected)
+
+
+class TestDeclaredFieldsConflict(TestCase):
+    def test_extra_kwargs_conflict(self):
+        class TestSerializer(serializers.ModelSerializer):
+            some_field = serializers.CharField()
+
+            class Meta:
+                model = OneFieldModel
+                extra_kwargs = {'some_field': {'read_only': True}}
+        with self.assertRaises(AssertionError):
+            TestSerializer().get_fields()
+
+    def test_read_only_fields_conflict(self):
+        class TestSerializer(serializers.ModelSerializer):
+            some_field = serializers.CharField()
+
+            class Meta:
+                model = OneFieldModel
+                read_only_fields = ('some_field', )
+        with self.assertRaises(AssertionError):
+            TestSerializer().get_fields()
+
+
+class TestUniquenessOverride(TestCase):
+    def test_required_not_overwritten(self):
+        class TestModel(models.Model):
+            field_1 = models.IntegerField(null=True)
+            field_2 = models.IntegerField()
+
+            class Meta:
+                unique_together = (('field_1', 'field_2'),)
+
+        class TestSerializer(serializers.ModelSerializer):
+            class Meta:
+                model = TestModel
+                extra_kwargs = {'field_1': {'required': False}}
+
+        fields = TestSerializer().fields
+        self.assertFalse(fields['field_1'].required)
+        self.assertTrue(fields['field_2'].required)