core/modules/rest/src/Plugin/ResourceBase.php | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/core/modules/rest/src/Plugin/ResourceBase.php b/core/modules/rest/src/Plugin/ResourceBase.php
index 743674f..62e9781 100644
--- a/core/modules/rest/src/Plugin/ResourceBase.php
+++ b/core/modules/rest/src/Plugin/ResourceBase.php
@@ -120,6 +120,12 @@ public function routes() {
         : $canonical_path;
       $route = $this->getBaseRoute($path, $method);
 
+      // Allow POST requests to the GET route, so we can respond with a 409 per
+      // the HTTP spec when POSTing to an existing resource.
+      if ($method === 'GET' && in_array('POST', $methods, TRUE)) {
+        $route->setMethods(array_merge($route->getMethods(), ['POST']));
+      }
+
       // Note that '_format' and '_content_type_format' route requirements are
       // added in ResourceRoutes::getRoutesForResourceConfig().
       $collection->add("$route_name.$method", $route);