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import { createApp } from "vue";
import App from "./App.vue";
import TheButton from "./components/TheButton";
const app = createApp(App);
app.component("the-button", TheButton);
createApp(App).mount("#app");
app.mount("#app");
In this section you should not write the last line. Because you already mounted the app, in the after last line
Source:stackexchange.com