Why is ngOnInit called twice?

if you used platformBrowserDynamic().bootstrapModule(AppModule); in app.module.ts comment it and try. I had the same problem. I think this helps

Why it is called twice

Right now, if an error happens during detecting changes of content/view children of a component, ngOnInit will be called twice (seen in DynamicChangeDetector). This can lead to follow up errors that hide the original error.

This information comes from this github issue

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So it seems that your mistake might have an origin elsewhere in your code, related to this component.

This was happening to me because of a faulty component html. I had forget to close the selector tag in the host component. So I had this <search><search>, instead of <search></search> - take note of the syntax error.

So related to @dylan answer, check your component html structure and that of its parent.