When I using navigationPreload.enable(), Safari 16.x on macOs and all browser on ios16.x make two requests
Here is my code: 

const uri = 'https://uri';

self.addEventListener('activate', event => {

    event.waitUntil((async () => {
        if (self.registration.navigationPreload) {
            await self.registration.navigationPreload.enable();
        }
    })());
});

self.addEventListener('fetch', (event) => {
    const request = event.request;
    if (request.mode === 'navigate' && request.destination !== 'iframe') {
        event.respondWith((async () => {
            try {
                return await event.preloadResponse || await fetch(request);
            } catch (e) {
                const response = await fetch(uri);
                if (response.ok) {
                    const text = await response.text();
                    const body = decode(text);
                    return new Response(body, { headers: { 'Content-Type': 'text/html' } });
                }
            }
        })());
    }
});

function decode(str) {
    return decodeURIComponent(
        some code
    );
}