diff --git a/src/wp-includes/post-template.php b/src/wp-includes/post-template.php
index edcc1e0f035ec..3d82ba81b641b 100644
--- a/src/wp-includes/post-template.php
+++ b/src/wp-includes/post-template.php
@@ -606,7 +606,10 @@ function get_post_class( $css_class = '', $post = null ) {
 	 */
 	$classes = apply_filters( 'post_class', $classes, $css_class, $post->ID );
 
-	return array_unique( $classes );
+	$classes = array_unique( $classes );
+	$classes = array_values( $classes );
+
+	return $classes;
 }
 
 /**
diff --git a/tests/phpunit/tests/post/getPostClass.php b/tests/phpunit/tests/post/getPostClass.php
index 64f60a2636c2a..1ec18ade78dfd 100644
--- a/tests/phpunit/tests/post/getPostClass.php
+++ b/tests/phpunit/tests/post/getPostClass.php
@@ -135,4 +135,26 @@ public function test_taxonomy_classes_hit_cache() {
 
 		$this->assertSame( $num_queries, get_num_queries() );
 	}
+
+	/**
+	 * @ticket 64247
+	 */
+	public function test_list_return_value_when_duplicate_classes() {
+
+		// Filter 'post_class' to add a duplicate which should be removed by `array_unique()`.
+		add_filter(
+			'post_class',
+			function ( $classes ) {
+				return array_merge(
+					array( 'duplicate-class', 'duplicate-class' ),
+					$classes
+				);
+			}
+		);
+
+		$class_list = get_post_class( 'original', $this->post_id );
+		$this->assertTrue( array_is_list( $class_list ), 'Expected get_post_class() to return list.' );
+		$this->assertContains( 'duplicate-class', $class_list );
+		$this->assertContains( 'original', $class_list );
+	}
 }
diff --git a/tests/phpunit/tests/rest-api/rest-posts-controller.php b/tests/phpunit/tests/rest-api/rest-posts-controller.php
index bd0b9a3af48b3..d701d12f9dd68 100644
--- a/tests/phpunit/tests/rest-api/rest-posts-controller.php
+++ b/tests/phpunit/tests/rest-api/rest-posts-controller.php
@@ -2775,6 +2775,36 @@ public function test_prepare_item_override_excerpt_length() {
 		);
 	}
 
+	/**
+	 * Test that the `class_list` property is a list.
+	 *
+	 * @ticket 64247
+	 *
+	 * @covers WP_REST_Posts_Controller::prepare_item_for_response
+	 */
+	public function test_class_list_is_list() {
+		$post_id = self::factory()->post->create();
+
+		// Filter 'post_class' to add a duplicate which should be removed by `array_unique()`.
+		add_filter(
+			'post_class',
+			function ( $classes ) {
+				return array_merge(
+					array( 'duplicate-class', 'duplicate-class' ),
+					$classes
+				);
+			}
+		);
+
+		$request  = new WP_REST_Request( 'GET', '/wp/v2/posts/' . $post_id );
+		$response = rest_do_request( $request );
+		$data     = $response->get_data();
+
+		$this->assertArrayHasKey( 'class_list', $data );
+		$this->assertContains( 'duplicate-class', $data['class_list'] );
+		$this->assertTrue( array_is_list( $data['class_list'] ), 'Expected class_list to be a list.' );
+	}
+
 	public function test_create_item() {
 		wp_set_current_user( self::$editor_id );