Reported as: https://github.com/mozilla/geckodriver/issues/1678

A simple Marionette testcase shows that behavior:

    def test_testcase_users(self):
        self.marionette.navigate("https://example.com/#test")
        self.marionette.navigate("https://google.com/#test")
        self.assertEqual(self.marionette.get_url(), "https://google.com/#test")

While we are waiting for the page load events for the first invocation, there is no event check at all for the second one:

1580162822981	Marionette	DEBUG	3 -> [0,2,"WebDriver:Navigate",{"url":"https://example.com/#test"}]
1580162822985	Marionette	TRACE	[6] Received DOM event beforeunload for about:blank
1580162823361	Marionette	TRACE	[6] Received DOM event pagehide for about:blank
1580162823370	Marionette	TRACE	[6] Received DOM event DOMContentLoaded for https://example.com/#test
1580162823537	Marionette	TRACE	[6] Received DOM event pageshow for https://example.com/#test
1580162823538	Marionette	DEBUG	3 <- [1,2,null,{"value":null}]
1580162823541	Marionette	DEBUG	3 -> [0,3,"WebDriver:Navigate",{"url":"https://google.com/#test"}]
1580162823561	Marionette	DEBUG	3 <- [1,3,null,{"value":null}]

I would like to see that fixed when continue working on bug 1612831.

Actually for back and forward this is too hard to do right now. We should wait until all the navigation happens in the parent process.