duplicate toast notifications after redirecting to a page with deferred props

[ericamigo]

i have an "access account" feature where i can impersonate the user then redirects to the accessed user's dashboard. the dashboard uses deferred props to load chart data. when accessing an account, i add a session success message for a toast notification using toastification. however, the toast notification appears twice - once on the initial page load and again after the deferred props are fetched.

my HandleInertiaRequest middleware for passing flash session to front-end via shared() method

'flash' => fn () => [
    'success' => $request->session()->get('success'),
    'error' => $request->session()->get('error'),
],

toastificiation in app.js

router.on('success', (event) => {
    const toast = useToast()

    if (event.detail.page.props.flash.success) {
        toast.success(event.detail.page.props.flash.success)
    }

    if (event.detail.page.props.flash.error) {
        toast.error(event.detail.page.props.flash.error, {
            timeout: false,
        })
    }
})

component in user's dashboard that uses deferred props

<Enquiries :filters="enquiriesChartFilters">
    <Deferred data="enquiriesChartData">
        <template #fallback></template>
        <EnquiriesGraph :data="enquiriesChartData" />
    </Deferred>
</Enquiries>

my controller

class UserImpersonateController extends Controller
{
    public function __invoke(User $user): RedirectResponse
    {
        Session::put('impersonating', Auth::user());

        Auth::login($user);

        return Redirect::route('dashboard')
            ->with('success', sprintf(
                'You are now logged in as %s',
                $user->name
            ));
    }
}

i tried console.log(event.detail.page.props.flash.success) in app.js, and it does appear twice. this is the second time i've encountered this issue. i'm not sure if it's inertia.js-related, but i noticed it only happens in this specific situation - after redirecting to a page that uses both deferred props and a flash session.