diff --git a/.changeset/sharp-snakes-poke.md b/.changeset/sharp-snakes-poke.md
new file mode 100644
index 0000000000..7f7f8aa7b2
--- /dev/null
+++ b/.changeset/sharp-snakes-poke.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: emit `each_key_duplicate` error in production
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js
index 1dbc34fdc3..b2724fa90f 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/EachBlock.js
@@ -338,10 +338,6 @@ export function EachBlock(node, context) {
 
 	const statements = [add_svelte_meta(b.call('$.each', ...args), node, 'each')];
 
-	if (dev && node.metadata.keyed) {
-		statements.unshift(b.stmt(b.call('$.validate_each_keys', thunk, key_function)));
-	}
-
 	if (node.metadata.expression.is_async()) {
 		context.state.init.push(
 			b.stmt(
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 6eaeac0f38..25f7cf91eb 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -40,6 +40,7 @@ import { get } from '../../runtime.js';
 import { DEV } from 'esm-env';
 import { derived_safe_equal } from '../../reactivity/deriveds.js';
 import { current_batch } from '../../reactivity/batch.js';
+import * as e from '../../errors.js';
 
 // When making substantive changes to this file, validate them with the each block stress test:
 // https://svelte.dev/playground/1972b2cf46564476ad8c8c6405b23b7b
@@ -290,6 +291,15 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 			}
 		}
 
+		if (length > keys.size) {
+			if (DEV) {
+				validate_each_keys(array, get_key);
+			} else {
+				// in prod, the additional information isn't printed, so don't bother computing it
+				e.each_key_duplicate('', '', '');
+			}
+		}
+
 		// remove excess nodes
 		if (hydrating && length > 0) {
 			set_hydrate_node(skip_nodes());
@@ -676,3 +686,30 @@ function link(state, prev, next) {
 		next.prev = prev;
 	}
 }
+
+/**
+ * @param {Array<any>} array
+ * @param {(item: any, index: number) => string} key_fn
+ * @returns {void}
+ */
+function validate_each_keys(array, key_fn) {
+	const keys = new Map();
+	const length = array.length;
+
+	for (let i = 0; i < length; i++) {
+		const key = key_fn(array[i], i);
+
+		if (keys.has(key)) {
+			const a = String(keys.get(key));
+			const b = String(i);
+
+			/** @type {string | null} */
+			let k = String(key);
+			if (k.startsWith('[object ')) k = null;
+
+			e.each_key_duplicate(a, b, k);
+		}
+
+		keys.set(key, i);
+	}
+}
diff --git a/packages/svelte/src/internal/client/index.js b/packages/svelte/src/internal/client/index.js
index 1d9f7dfff7..7fcaf77dc5 100644
--- a/packages/svelte/src/internal/client/index.js
+++ b/packages/svelte/src/internal/client/index.js
@@ -158,7 +158,7 @@ export {
 	deep_read_state,
 	active_effect
 } from './runtime.js';
-export { validate_binding, validate_each_keys } from './validate.js';
+export { validate_binding } from './validate.js';
 export { raf } from './timing.js';
 export { proxy } from './proxy.js';
 export { create_custom_element } from './dom/elements/custom-element.js';
diff --git a/packages/svelte/src/internal/client/validate.js b/packages/svelte/src/internal/client/validate.js
index 48a44db304..a169225f1e 100644
--- a/packages/svelte/src/internal/client/validate.js
+++ b/packages/svelte/src/internal/client/validate.js
@@ -1,45 +1,11 @@
 /** @import { Blocker } from '#client' */
 import { dev_current_component_function } from './context.js';
-import { is_array } from '../shared/utils.js';
-import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
 import { render_effect } from './reactivity/effects.js';
 import * as w from './warnings.js';
 import { capture_store_binding } from './reactivity/store.js';
 import { run_after_blockers } from './reactivity/async.js';
 
-/**
- * @param {() => any} collection
- * @param {(item: any, index: number) => string} key_fn
- * @returns {void}
- */
-export function validate_each_keys(collection, key_fn) {
-	render_effect(() => {
-		const keys = new Map();
-		const maybe_array = collection();
-		const array = is_array(maybe_array)
-			? maybe_array
-			: maybe_array == null
-				? []
-				: Array.from(maybe_array);
-		const length = array.length;
-		for (let i = 0; i < length; i++) {
-			const key = key_fn(array[i], i);
-			if (keys.has(key)) {
-				const a = String(keys.get(key));
-				const b = String(i);
-
-				/** @type {string | null} */
-				let k = String(key);
-				if (k.startsWith('[object ')) k = null;
-
-				e.each_key_duplicate(a, b, k);
-			}
-			keys.set(key, i);
-		}
-	});
-}
-
 /**
  * @param {string} binding
  * @param {Blocker[]} blockers
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/_config.js b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/_config.js
new file mode 100644
index 0000000000..bc945876ea
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/_config.js
@@ -0,0 +1,12 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		let button = target.querySelector('button');
+
+		button?.click();
+
+		assert.throws(flushSync, 'https://svelte.dev/e/each_key_duplicate');
+	}
+});
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/main.svelte b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/main.svelte
new file mode 100644
index 0000000000..f8ba50d866
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-2/main.svelte
@@ -0,0 +1,8 @@
+<script>
+	let data = $state([1, 2, 3]);
+</script>
+
+<button onclick={() => data = [1, 1, 1]}>add</button>
+{#each data as d (d)}
+	{d}
+{/each}
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/_config.js b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/_config.js
new file mode 100644
index 0000000000..7e1840200a
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/_config.js
@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	error: 'each_key_duplicate'
+});
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/main.svelte b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/main.svelte
new file mode 100644
index 0000000000..a05781bcb9
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique-3/main.svelte
@@ -0,0 +1,7 @@
+<script>
+	let data = [1, 1, 1];
+</script>
+
+{#each data as d (d)}
+	{d}
+{/each}
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique/_config.js b/packages/svelte/tests/runtime-production/samples/keyed-each-unique/_config.js
new file mode 100644
index 0000000000..bc945876ea
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique/_config.js
@@ -0,0 +1,12 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		let button = target.querySelector('button');
+
+		button?.click();
+
+		assert.throws(flushSync, 'https://svelte.dev/e/each_key_duplicate');
+	}
+});
diff --git a/packages/svelte/tests/runtime-production/samples/keyed-each-unique/main.svelte b/packages/svelte/tests/runtime-production/samples/keyed-each-unique/main.svelte
new file mode 100644
index 0000000000..3d52179372
--- /dev/null
+++ b/packages/svelte/tests/runtime-production/samples/keyed-each-unique/main.svelte
@@ -0,0 +1,8 @@
+<script>
+	let data = $state([1, 2, 3, 4, 5]);
+</script>
+
+<button onclick={() => data = [1, 1, 3, 1]}>add</button>
+{#each data as d (d)}
+	{d}
+{/each}