Async guard executed twice when navigating back

[pawelbialaszczyk]

Background

Whenever there's an async result returned from a guard (i.e. an Observable) attached to a route that's being navigated back to (i.e. by clicking the browser's back button), that guard is executed twice (first navigation event gets canceled). This results in breaking the browsing history.

Demonstration

I've pushed a minimal example forked from the official template: https://github.com/pawelbialaszczyk/coexisting-angular-microfrontends. Used the latest single-spa, single-spa-angular and angular versions.

Repro steps:

1. Start app1 locally
2. Navigate to http://coexisting-angular-microfrontends.surge.sh
3. Override app1 module URL to use the local one
4. Open the browser's dev tools (console view)
5. Click App1
6. Click Go to test-1 (notice in console the guard is executed once)
7. Click Go to test-2
8. Click Go back (notice in console the guard is executed twice)

Expected Behavior

When clicking Go back the guard is executed once

Actual Behavior

When clicking Go back the guard is executed twice