fix: handle duplicate signal dependencies gracefully

.changeset/witty-phones-retire.md

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

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

@@ -417,16 +417,21 @@ function remove_reaction(signal, dependency) {
  * @returns {void}
  */
 export function remove_reactions(signal, start_index) {
-	const dependencies = signal.deps;
-	if (dependencies !== null) {
-		const active_dependencies = start_index === 0 ? null : dependencies.slice(0, start_index);
-		let i;
-		for (i = start_index; i < dependencies.length; i++) {
-			const dependency = dependencies[i];
-			// Avoid removing a reaction if we know that it is active (start_index will not be 0)
-			if (active_dependencies === null || !active_dependencies.includes(dependency)) {
-				remove_reaction(signal, dependency);
-			}
+	var dependencies = signal.deps;
+	if (dependencies === null) return;
+
+	var active_dependencies = start_index === 0 ? null : dependencies.slice(0, start_index);
+	var seen = new Set();
+
+	for (var i = start_index; i < dependencies.length; i++) {
+		var dependency = dependencies[i];
+
+		if (seen.has(dependency)) continue;
+		seen.add(dependency);
+
+		// Avoid removing a reaction if we know that it is active (start_index will not be 0)
+		if (active_dependencies === null || !active_dependencies.includes(dependency)) {
+			remove_reaction(signal, dependency);
 		}
 	}
 }
@@ -774,10 +779,7 @@ export function get(signal) {
 		) {
 			if (current_dependencies === null) {
 				current_dependencies = [signal];
-			} else if (
-				current_dependencies[current_dependencies.length - 1] !== signal &&
-				!current_dependencies.includes(signal)
-			) {
+			} else if (current_dependencies[current_dependencies.length - 1] !== signal) {
 				current_dependencies.push(signal);
 			}
 		}

packages/svelte/tests/runtime-runes/samples/derived-dependencies-2/_config.js

@@ -0,0 +1,37 @@
+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>toggle a</button>
+				<button>toggle b</button>
+				false/true/true
+			`
+		);
+
+		flushSync(() => btn2?.click());
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>toggle a</button>
+				<button>toggle b</button>
+			`
+		);
+
+		flushSync(() => btn2?.click());
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>toggle a</button>
+				<button>toggle b</button>
+				false/true/true
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-dependencies-2/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	let a = $state(true);
+	let b = $state({ c: true });
+
+	const x = $derived(b);
+</script>
+
+<button onclick={() => (a = !a)}>toggle a</button>
+<button onclick={() => (b = b ? null : { c: true })}>toggle b</button>
+
+{#if x}
+	{a}/{x.c}/{x.c}
+{/if}