Do not try to generate fragments for generated fragments

core-bundle/src/DependencyInjection/Compiler/RegisterFragmentsPass.php

@@ -83,6 +83,14 @@ protected function registerFragments(ContainerBuilder $container, string $tag):
         $command = $container->hasDefinition('contao.command.debug_fragments') ? $container->findDefinition('contao.command.debug_fragments') : null;
 
         foreach ($this->findAndSortTaggedServices($tag, $container) as $reference) {
+            // If a controller has multiple methods for different fragment type (e.g. a content element
+            // and a front end module), the first RegisterFragmentsPass creates a child definition that
+            // inherits all tags from the original. On the next run, the RegisterFragmentsPass would
+            // pick up the child definition and try to create duplicate fragments.
+            if (0 === strpos((string) $reference, 'contao.fragment._')) {
+                continue;
+            }
+
             $definition = $container->findDefinition($reference);
 
             $tags = $definition->getTag($tag);