Multiple workers / consumers are consuming the same message twice?

[MrSnoozles]

Hi,

I noticed that when I start multiple consumers the same messages are executed by the consumers. Is there a way to "reserve" a message? I'm using Dbal to store the messages in a MariaDB database.

How I dispatch the messages

$queue = $context->createQueue('default');
$message = $context->createMessage($messageContent);
$producer = $context->createProducer();
$producer->send($queue, $message);

How I consume the messages

// bin/start-worker.php

/* @var $context DbalContext */
$subscriptionConsumer = $context->createSubscriptionConsumer();
$queue    = $context->createQueue('default');
$consumer = $context->createConsumer($queue);

$subscriptionConsumer->subscribe(
    $consumer,
    function (Message $message, Consumer $consumer) {
        $job = unserialize($message->getBody());
        $job->handle();

        $consumer->acknowledge($message);
    }
);

$subscriptionConsumer->consume(0);

Now running php bin/start-worker.php multiple times will lead to the same messages being picked up several times. Am I understanding something wrong?