diff --git a/.travis.yml b/.travis.yml
index 680e571..c064a3c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,13 +1,20 @@
 sudo: required
 
 language: php
-php:
-  - 7.2
-  - 7.3
-  - 7.4
+
+jobs:
+  include:
+    # Combination PHP <7.4 and maglnet/composer-require-checker doesn't work with Composer 2
+    - php: 7.2
+      env: COMPOSER_VERSION=1.10.16
+    - php: 7.3
+      env: COMPOSER_VERSION=1.10.16
+    - php: 7.4
+      env: COMPOSER_VERSION=--stable
 
 env:
-  - ESB_CONSOLE_PORT=8080 ESB_HTTP_SERVER_PORT=34981 ESB_BEANSTALKD_URL=tcp://127.0.0.1:11300 ES_BASE_URI=http://127.0.0.1:9200
+  global:
+    - ESB_CONSOLE_PORT=8080 ESB_HTTP_SERVER_PORT=34981 ESB_BEANSTALKD_URL=tcp://127.0.0.1:11300 ES_BASE_URI=http://127.0.0.1:9200
 
 cache:
   directories:
@@ -24,6 +31,8 @@ before_install:
   - echo -e '-Ddiscovery.type=single-node\n-XX:+DisableExplicitGC\n-Djdk.io.permissionsUseCanonicalPath=true\n-Dlog4j.skipJansi=true\n-server\n' | sudo tee -a /etc/elasticsearch/jvm.options
   - sudo chown -R elasticsearch:elasticsearch /etc/default/elasticsearch
   - sudo systemctl start elasticsearch
+  - composer --verbose self-update $COMPOSER_VERSION
+  - composer --version
 
 install:
   - sudo apt-get update
@@ -31,7 +40,7 @@ install:
 
 before_script:
   - composer install
-  - composer global require maglnet/composer-require-checker && $HOME/.composer/vendor/bin/composer-require-checker --config-file=composer-require-checker.json;
+  - composer global require maglnet/composer-require-checker && $(composer config home)/vendor/bin/composer-require-checker --config-file=composer-require-checker.json;
   - vendor/bin/ecs check
   - vendor/bin/phpstan analyse --no-progress -l max -c phpstan.neon src/
 
diff --git a/composer.json b/composer.json
index f4721fb..8ab1a28 100644
--- a/composer.json
+++ b/composer.json
@@ -56,8 +56,8 @@
         "pda/pheanstalk": "^3.1",
         "mikey179/vfsstream": "^1.6",
         "amphp/artax": "^3.0",
-        "phpstan/phpstan": "^0.12",
-        "symplify/easy-coding-standard-prefixed": "^8.1"
+        "phpstan/phpstan": "^0.12 <=0.12.66",
+        "symplify/easy-coding-standard-prefixed": "^8.1 <8.3"
     },
     "scripts": {
         "phpcs": "phpcs",
diff --git a/src/Exception/ElasticSearch/JobNotFoundException.php b/src/Exception/ElasticSearch/JobNotFoundException.php
index f1f16ff..7566146 100644
--- a/src/Exception/ElasticSearch/JobNotFoundException.php
+++ b/src/Exception/ElasticSearch/JobNotFoundException.php
@@ -8,7 +8,7 @@
 
 class JobNotFoundException extends \RuntimeException
 {
-    public function __construct(string $jobUuid, $code = 0, Throwable $previous = null)
+    public function __construct(string $jobUuid, int $code = 0, Throwable $previous = null)
     {
         parent::__construct(sprintf('Job with UUID "%s" has not been found.', $jobUuid), $code, $previous);
     }
diff --git a/src/Service/QueueManager.php b/src/Service/QueueManager.php
index 505934e..b90ce29 100644
--- a/src/Service/QueueManager.php
+++ b/src/Service/QueueManager.php
@@ -223,9 +223,16 @@ private function jobExists(string $jobUuid): Promise
     private function processBatch(): \Generator
     {
         $this->logger->debug('Processing batch');
-        yield $this->elasticSearch->bulkIndexJobs($this->batch, $this->flowConfig->getTube());
 
-        foreach ($this->batch as $singleJob) {
+        // Store batch in a local variable and clear the shared property. Any other Producer instance may update and
+        // process $this->batch during a yield call within this method (emulating concurrency). This shouldn't cause
+        // duplicate inserts of the same job.
+        $batch = $this->batch;
+        $this->batch = [];
+
+        yield $this->elasticSearch->bulkIndexJobs($batch, $this->flowConfig->getTube());
+
+        foreach ($batch as $singleJob) {
             yield $this->beanstalkClient->put(
                 $singleJob->getUuid(),
                 $singleJob->getTimeout(),
@@ -233,8 +240,6 @@ private function processBatch(): \Generator
                 $singleJob->getPriority()
             );
         }
-
-        $this->batch = [];
     }
 
     /**