.../tests/src/Functional/EntityResource/EntityResourceTestBase.php   | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index 2de3852..133e8d2 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -869,6 +869,11 @@ public function testPost() {
     }
 
 
+    // DX: 409 when POSTing to an existing entity.
+    $response = $this->request('POST', $this->getEntityResourceUrl()->setOption('query', ['_format' => static::$format]), $request_options);
+    $this->assertResourceErrorResponse(409, 'POSTING to an already existing entity resource', $response);
+
+
     $this->config('rest.settings')->set('bc_entity_resource_permissions', TRUE)->save(TRUE);
     $this->refreshTestStateAfterRestConfigChange();
     $request_options[RequestOptions::BODY] = $parseable_valid_request_body_2;