fix: emit `each_key_duplicate` error in production

.changeset/sharp-snakes-poke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: emit `each_key_duplicate` error in production

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -42,6 +42,7 @@ import { active_effect, 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 { each_key_duplicate } from '../../errors.js';
 
 /**
  * The row of a keyed each block that is currently updating. We track this
@@ -473,6 +474,21 @@ function reconcile(
 					var start = stashed[0];
 					var j;
 
+					// full key uniqueness check is dev-only,
+					// key duplicates cause crushing only due to `matched` being empty
+					if (matched.length === 0) {
+						var map = new Map();
+						for (j = 0; j < length; j += 1) {
+							var k = get_key(array[j], j);
+							if (map.has(k)) {
+								k = String(k);
+								if (k.startsWith('[object ')) k = null;
+								each_key_duplicate(String(j), String(map.get(k)), k);
+							}
+							map.set(k, j);
+						}
+					}
+
 					prev = start.prev;
 
 					var a = matched[0];

packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/_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, /each_key_duplicate/);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/keyed-each-dev-unique-prod/main.svelte

@@ -0,0 +1,8 @@
+<script>
+	let data = [1, 2, 3];
+</script>
+
+<button onclick={() => data = [1, 1, 3, 1]}>add</button>
+{#each data as d (d)}
+	{d}
+{/each}