I got this error because I somehow accidentally imported the root AppModule into my lazy-loaded module. This caused the root AppRoutingModule to be called again when the lazy-loaded module was loaded, and thus RouterModule.forRoot was called twice.
If you are certain you aren’t calling RouterModule.forRoot twice then this could be the cause of the issue. Check that you aren’t importing any modules in your lazy-loaded module that direcly call RouterModule.forRoot or import a module that calls RouterModule.forRoot.