'ng serve' does not work after a double install

I used to have Angular CLI, but I mistakenly installed it again with

npm install -g angular-cli


And now when I run ng serve it complains:

It seems like you're using a project generated using an old version of the Angular CLI.

The latest CLI now uses webpack and includes a lot of improvements, include a simpler workflow, a faster build and smaller bundles.

To get more info, including a step-by-step guide to upgrade the CLI, follow this link: https://github.com/angular/angular-cli/wiki/Upgrading-from-Beta.10-to-Beta.12
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When I try to create a new project using ng new in another directory, it complains that:

Cannot find module 'reflect-metadata'
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So what is the problem?

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