Python: Option to validate duplicate fields during json deserialization

[sersh4nt]

The issue

Proto:

syntax = "proto3";

package test;

message User {
  string first_name = 1 [json_name = "first_name"];
  string last_name = 2;
}

Code:

from google.protobuf.json_format import ParseDict
import test_pb2

data = {"first_name": "Aaron", "last_name": "Surname", "lastName": "Fake"}

message = ParseDict(data, test_pb2.User())

print(message)

Output:

first_name: "Aaron"
last_name: "Fake"

Why is this a problem?

This is a pretty unexpected behavior. Even using json_name cannot fully prevent one from using duplicate fields in json.

What about implementations in other languages?

protobuf-go features a specific check for duplicate fields.
This PR is created similarly to C# PR

What does this PR do?

This PR improves checks for fields already used during JSON parsing. Back to the example, the result will be as follows:

Code:

from google.protobuf.json_format import ParseDict
import test_pb2

data = {"first_name": "Aaron", "last_name": "Surname", "lastName": "Fake"}

message = ParseDict(data, test_pb2.User())

print(message)

Output:

Traceback (most recent call last):
  File "/Users/sersh4nt/Projects/protobuf/venv/lib/python3.12/site-packages/google/protobuf/json_format.py", line 597, in _ConvertFieldValuePair
    raise ParseError(
google.protobuf.json_format.ParseError: Message type "test.User" should not have multiple "lastName" fields at "User".