emit error on hydration

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

@@ -273,6 +273,8 @@ export function each(node, flags, get_collection, get_key, render_fn, fallback_f
 		if (hydrating) {
 			if (length === 0 && fallback_fn) {
 				fallback = branch(() => fallback_fn(anchor));
+			} else if (length > state.items.size) {
+				each_key_duplicate('', '', '');
 			}
 		} else {
 			if (should_defer_append()) {

packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-3/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: false
+	},
+	error: 'each_key_duplicate\nKeyed each block has duplicate key at indexes  and '
+});

packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-3/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let data = [1, 1, 1];
+</script>
+
+{#each data as d (d)}
+	{d}
+{/each}