@Test
    public void testCorrectHandlingOfDuplicateSequenceError() throws Exception {
        final long producerId = 343434L;
        TransactionManager transactionManager = createTransactionManager();
        setupWithTransactionState(transactionManager);
        prepareAndReceiveInitProducerId(producerId, Errors.NONE);
        assertTrue(transactionManager.hasProducerId());

        assertEquals(0, transactionManager.sequenceNumber(tp0));

        // Send first ProduceRequest
        Future<RecordMetadata> request1 = appendToAccumulator(tp0);
        sender.runOnce();
        String nodeId = client.requests().peek().destination();
        Node node = new Node(Integer.parseInt(nodeId), "localhost", 0);
        assertEquals(1, client.inFlightRequestCount());
        assertEquals(1, transactionManager.sequenceNumber(tp0));
        assertEquals(OptionalInt.empty(), transactionManager.lastAckedSequence(tp0));

        // Send second ProduceRequest
        Future<RecordMetadata> request2 = appendToAccumulator(tp0);
        sender.runOnce();
        assertEquals(2, client.inFlightRequestCount());
        assertEquals(2, transactionManager.sequenceNumber(tp0));
        assertEquals(OptionalInt.empty(), transactionManager.lastAckedSequence(tp0));
        assertFalse(request1.isDone());
        assertFalse(request2.isDone());
        assertTrue(client.isReady(node, time.milliseconds()));

        ClientRequest firstClientRequest = client.requests().peek();
        ClientRequest secondClientRequest = (ClientRequest) client.requests().toArray()[1];

        client.respondToRequest(secondClientRequest, produceResponse(tp0, 1000, Errors.NONE, 0));

        sender.runOnce(); // receive response 1

        assertEquals(OptionalLong.of(1000), transactionManager.lastAckedOffset(tp0));
        assertEquals(OptionalInt.of(1), transactionManager.lastAckedSequence(tp0));

        client.respondToRequest(firstClientRequest, produceResponse(tp0, ProduceResponse.INVALID_OFFSET, Errors.DUPLICATE_SEQUENCE_NUMBER, 0));

        sender.runOnce(); // receive response 0

        // Make sure that the last ack'd sequence doesn't change.
        assertEquals(OptionalInt.of(1), transactionManager.lastAckedSequence(tp0));
        assertEquals(OptionalLong.of(1000), transactionManager.lastAckedOffset(tp0));
        assertFalse(client.hasInFlightRequests());

        RecordMetadata unknownMetadata = request1.get();
        assertFalse(unknownMetadata.hasOffset());
        assertEquals(-1L, unknownMetadata.offset());
    }
Tests the code path where the target node to send FindCoordinator or InitProducerId
is not ready.
clients/src/test/java/org/apache/kafka/clients/producer/internals/SenderTest.java