Q:

Angular multiple imports

[EDIT]: My bad, I'm a bit tired and I've mixed two ideas in my mind resulting in a question which doesn't really mean much ... I'll blame it on the coffee! :P 

This is more of a knowledge question rather than an actual problem, but...
I'm currently developing an angular web application and, as I started "messing" with tests I realized most of the tests fail because I didn't include the components in the component-name.ts file while i import them in app.module.ts, yet I call custom components. 
(E.G.: <app-menu></app-menu>  Or  <router-outlet></router-outlet>)

Now, the solution is pretty easy: just import the modules and it's done, but it made me wonder... performance wise is it worth to import them?
How does Angularjs deals with re-importing the same thing multiple times?
For instance: let's say I 
import { RouterModule } from '@angular/router'; in app.module.ts

and that then I 

import { RouterModule } from '@angular/router'; in ./menu/menu.module.ts

Does angular removes the second import in compile phase or does it simply ignores it?

A:

simply explained  here:
https://medium.com/@cyrilletuzi/understanding-angular-modules-ngmodule-and-their-scopes-81e4ed6f7407
you should not import multiple times a module, but it's necessary when you have to make that module visible to every sub-components -- and this is NOT a RE-import --

