Can´t add a silhouette to the `postProcessStages`

I want to reproduce the simple example as shown in the Cesium silhouette example. But I am not able to add a silhouette and receive the error message:

DeveloperError {name: ‘DeveloperError’, message: ‘czm_silhouette has already been added to the collection or does not have a unique name.’, stack: ‘Error\n at new DeveloperError (/static/js/bundle.js:78893:11)’}

Here is my code snippet:

import { PostProcessStageLibrary } from "cesium";
import { useCesium } from "resium";

export default function TestSilhouette() {
    const Cesium = useCesium();
    const viewer = Cesium.viewer;

    const silhouetteBlue = PostProcessStageLibrary.createEdgeDetectionStage();
    silhouetteBlue.uniforms.color = Color.BLUE;
    silhouetteBlue.uniforms.length = 0.01;
    silhouetteBlue.selected = [];

    const silhouetteGreen = PostProcessStageLibrary.createEdgeDetectionStage();
    silhouetteGreen.uniforms.color = Color.LIME;
    silhouetteGreen.uniforms.length = 0.01;
    silhouetteGreen.selected = [];



Furthermore, I can find nothing related to this error message.