JS WebSocket client callbacks invoked twice on parallel requests

When using a WebSocket Transport and doing two service calls immediately, without waiting for the first to return, e.g. like this:

const t = new Thrift.TWebSocketTransport('ws://localhost:8338/hello');
	const p = new Thrift.TJSONProtocol(t);
	const client = new HelloSvcClient(p);

	t.open();

	client.test(function (res) {
		console.log(1, res);
	});
	client.test(function (res) {
		console.log(2, res);
	});

The callback to the first invocation is called twice, and the second never, i.e. console shows:

1 "test result"
1 "test result"

instead of the expected

1 "test result"
2 "test result"

I suspect this bug was introduced with the patch for https://issues.apache.org/jira/browse/THRIFT-4131 where for some reason the callback registered twice when set: https://github.com/apache/thrift/pull/1372/files