Reported by Javier:

I'm updating Twig extensions to use the#[AsTwig...] attributes. I removed thegetFunctions() method and had this:

class FooExtension extends AbstractExtension{    #[AsTwigFunction('name_of_function')]    public function foo(): string    {        // ...    }}

Then I saw this exception:Unable to register extension "App\Twig\FooExtension" as it is already registered.
My error is that I forgot to remove extendsAbstractExtension
But I think that Symfony's error exception should be more helpful. A quick example of something that could've helped me:"When using the AsTwigFunction attribute, your Twig extension cannot extend AbstractExtension"

The ExtensionSet exception is thrown because the same an extension is registered twice:

1. It has thetwig.extension tag autoconfigured because it implementsTwig\Extension\ExtensionInterface
2. AnAttributeExtension is registered for this class because it as one of the extension attributes (since[TwigBundle] Enable#[AsTwigFilter],#[AsTwigFunction] and#[AsTwigTest] attributes to configure runtime extensions #52748)

Previous exception whentwig service is instanciated:

LogicException : Unable to register extension "Symfony\Bundle\TwigBundle\Tests\Functional\InvalidExtensionWithAttributes" as it is already registered.

New exception during container compilation:

Symfony\Component\DependencyInjection\Exception\LogicException : The class "Symfony\Bundle\TwigBundle\Tests\Functional\InvalidExtensionWithAttributes" cannot both extend "Twig\Extension\AbstractExtension" and use the "#[Twig\Attribute\AsTwigFilter]" attribute on method "funFilter()", choose one or the other.

CheckingAbstractExtension makes the error message more specific to the code, as most extensions extend this class and doesn't implement the interface directly.