diff --git a/src/FeaturesManager.php b/src/FeaturesManager.php
index 721d33a..1125425 100644
--- a/src/FeaturesManager.php
+++ b/src/FeaturesManager.php
@@ -498,7 +498,7 @@ class FeaturesManager implements FeaturesManagerInterface {
         $already_in_package = in_array($item_name, $package->getConfig());
         if (($force || (!$already_assigned && $assignable && !$excluded_from_package)) && !$already_in_package) {
           // Add the item to the package's config array.
-          $package['config'][] = $item_name;
+          $package->appendConfig($item_name);
           // Mark the item as already assigned.
           $config_collection[$item_name]->setPackage($package_name);
           // For configuration in the InstallStorage::CONFIG_INSTALL_DIRECTORY
diff --git a/src/Package.php b/src/Package.php
index 752141b..3f8e46a 100644
--- a/src/Package.php
+++ b/src/Package.php
@@ -175,6 +175,19 @@ class Package {
   }
 
   /**
+   * Append a new filename.
+   *
+   * @param string $config
+   *
+   * @return $this
+   */
+  public function appendConfig($config) {
+    $this->config[] = $config;
+    $this->config = array_unique($this->config);
+    return $this;
+  }
+
+  /**
    * @return string
    */
   public function getBundle() {
diff --git a/tests/src/Unit/FeaturesManagerTest.php b/tests/src/Unit/FeaturesManagerTest.php
index 5e9c101..57549ab 100644
--- a/tests/src/Unit/FeaturesManagerTest.php
+++ b/tests/src/Unit/FeaturesManagerTest.php
@@ -347,6 +347,24 @@ class FeaturesManagerTest extends UnitTestCase {
   }
 
   /**
+   * @covers ::assignConfigPackage
+   */
+  public function testAssignConfigPackage() {
+    $config_collection = [
+      'test_config' => new ConfigurationItem('test_config', []),
+      'test_config2' => new ConfigurationItem('test_config2', []),
+    ];
+    $this->featuresManager->setConfigCollection($config_collection);
+
+    $package = new Package('test_package');
+    $this->featuresManager->setPackage($package);
+
+    $this->featuresManager->assignConfigPackage('test_package', ['test_config', 'test_config2']);
+
+    $this->assertEquals(['test_config', 'test_config2'], $this->featuresManager->getPackage('test_package')->getConfig());
+  }
+
+  /**
    * @covers ::initPackageFromExtension
    * @covers ::getPackageObject
    */