core/modules/rest/src/Plugin/ResourceBase.php |  4 +++-
 core/modules/rest/src/RequestHandler.php      | 10 +++++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/core/modules/rest/src/Plugin/ResourceBase.php b/core/modules/rest/src/Plugin/ResourceBase.php
index e91afd9..a53208c 100644
--- a/core/modules/rest/src/Plugin/ResourceBase.php
+++ b/core/modules/rest/src/Plugin/ResourceBase.php
@@ -131,9 +131,11 @@ public function routes() {
             $format_route = clone $route;
             $format_route->addRequirements(['_format' => $format_name]);
 
+            // Also allow this method to handle POST requests, so we can send
+            // 409 responses for already-existing resources.
             $methods = $route->getMethods();
             $methods[] = 'POST';
-            $route->setMethods($methods);
+            $format_route->setMethods($methods);
             $collection->add("$route_name.$method.$format_name", $format_route);
           }
           break;
diff --git a/core/modules/rest/src/RequestHandler.php b/core/modules/rest/src/RequestHandler.php
index aea0509..9d04f98 100644
--- a/core/modules/rest/src/RequestHandler.php
+++ b/core/modules/rest/src/RequestHandler.php
@@ -74,7 +74,15 @@ public function handle(RouteMatchInterface $route_match, Request $request) {
     // @see \Symfony\Component\Routing\Matcher\UrlMatcher::matchCollection()
     // @see \Symfony\Component\HttpFoundation\Response::prepare()
     $method = strtolower($route_match->getRouteObject()->getMethods()[0]);
-    assert(count($route_match->getRouteObject()->getMethods()) === 1);
+    if ($method !== 'get') {
+      assert(count($route_match->getRouteObject()->getMethods()) === 1);
+    }
+    // REST resources' "GET" routes also support POST, to be able to throw a
+    // \Symfony\Component\HttpKernel\Exception\ConflictHttpException.
+    // @see \Drupal\rest\Plugin\ResourceBase::routes()
+    else {
+      assert($route_match->getRouteObject()->getMethods() === ['GET', 'POST']);
+    }
 
 
     $resource_config_id = $route_match->getRouteObject()->getDefault('_rest_resource_config');