fix: prevent duplicate <link> module preloads (#10442)

fixes #10358
fixes #10379
The path optimization was applied too often, causing relative/absolute path mismatches, causing Vite to add duplicate link module preloads

Author: teemingc

.changeset/funny-items-deny.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prevent duplicate module preload

packages/kit/src/exports/vite/index.js

@@ -545,7 +545,8 @@ function kit({ svelte_config }) {
 				// E.g. Vite generates `new URL('/asset.png', import.meta).href` for a relative path vs just '/asset.png'.
 				// That's larger and takes longer to run and also causes an HTML diff between SSR and client
 				// causing us to do a more expensive hydration check.
-				const client_base = kit.paths.relative || kit.paths.assets ? './' : kit.paths.base || '/';
+				const client_base =
+					kit.paths.relative !== false || kit.paths.assets ? './' : kit.paths.base || '/';
 
 				new_config = {
 					base: ssr ? assets_base(kit) : client_base,

packages/kit/test/apps/basics/test/client.test.js

@@ -802,3 +802,27 @@ test.describe('Actions', () => {
 		await expect(pre).toHaveText('prop: 1, store: 1');
 	});
 });
+
+test.describe('Assets', () => {
+	test('only one link per stylesheet', async ({ page }) => {
+		if (process.env.DEV) return;
+
+		await page.goto('/');
+
+		expect(
+			await page.evaluate(() => {
+				const links = Array.from(document.head.querySelectorAll('link[rel=stylesheet]'));
+
+				for (let i = 0; i < links.length; ) {
+					const link = links.shift();
+					const asset_name = link.href.split('/').at(-1);
+					if (links.some((link) => link.href.includes(asset_name))) {
+						return false;
+					}
+				}
+
+				return true;
+			})
+		).toBe(true);
+	});
+});