Prevent re-running deserialization. Closes #1975.

rest_framework/renderers.py

@@ -523,8 +523,10 @@ class BrowsableAPIRenderer(BaseRenderer):
                 data = request.data
             except ParseError:
                 data = None
+            existing_serializer = serializer
         else:
             data = None
+            existing_serializer = None
 
         with override_method(view, request, method) as request:
             if not self.show_form_for_method(view, method, request, instance):
@@ -539,6 +541,9 @@ class BrowsableAPIRenderer(BaseRenderer):
             ):
                 return
 
+            if existing_serializer is not None:
+                serializer = existing_serializer
+            else:
                 serializer = view.get_serializer(instance=instance, data=data)
                 if data is not None:
                     serializer.is_valid()