fix: avoid duplicate signal dependencies (#12245)

Fixes #12230

.changeset/polite-peas-mate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid duplicate signal dependencies

packages/svelte/src/internal/client/runtime.js

@@ -774,7 +774,10 @@ export function get(signal) {
 		) {
 			if (current_dependencies === null) {
 				current_dependencies = [signal];
-			} else if (current_dependencies[current_dependencies.length - 1] !== signal) {
+			} else if (
+				current_dependencies[current_dependencies.length - 1] !== signal &&
+				!current_dependencies.includes(signal)
+			) {
 				current_dependencies.push(signal);
 			}
 		}

packages/svelte/tests/runtime-runes/samples/derived-dependencies/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target }) {
+		let [btn1, btn2] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>hide</button><button>show</button`);
+
+		flushSync(() => {
+			btn2?.click();
+		});
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`<h1>John Doe</h1><p>Body</p><button>hide</button><button>show</button>`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-dependencies/main.svelte

@@ -0,0 +1,27 @@
+<script>
+	let data = $state({
+		event: {
+			author: 'John Doe',
+			body: 'Body'
+		}
+	});
+	const { event } = $derived(data);
+</script>
+
+{#if event}
+	<h1>{event.author}</h1>
+	<p>{event.body}</p>
+{/if}
+
+<button onclick={() => {
+	data = {}
+}}>hide</button>
+
+<button onclick={() => {
+	data = {
+			event: {
+				author: 'John Doe',
+				body: 'Body'
+			}
+		}
+}}>show</button>