diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 3d8e7c8870..bdf3f717f2 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -370,6 +370,7 @@ function reconcile(
 	var is_animated = (flags & EACH_IS_ANIMATED) !== 0;
 	var should_update = (flags & (EACH_ITEM_REACTIVE | EACH_INDEX_REACTIVE)) !== 0;
 
+	var count = 0;
 	var length = array.length;
 	var items = state.items;
 	var first = state.first;
@@ -458,6 +459,7 @@ function reconcile(
 			stashed = [];
 
 			current = prev.next;
+			count += 1;
 			continue;
 		}
 
@@ -519,6 +521,7 @@ function reconcile(
 					link(state, prev, item);
 
 					prev = item;
+					count += 1;
 				}
 
 				continue;
@@ -547,6 +550,14 @@ function reconcile(
 		matched.push(item);
 		prev = item;
 		current = item.next;
+		count += 1;
+	}
+
+	if (count !== length) {
+		// full key uniqueness check is dev-only,
+		// if keys duplication didn't cause a crash,
+		// the rendered list will be shorter then the array
+		each_key_duplicate('', '', '');
 	}
 
 	if (current !== null || seen !== undefined) {
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/_config.js b/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/_config.js
new file mode 100644
index 0000000000..4caa45fd3c
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/_config.js
@@ -0,0 +1,15 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: false
+	},
+	test({ assert, target }) {
+		let button = target.querySelector('button');
+
+		button?.click();
+
+		assert.throws(flushSync, /each_key_duplicate/);
+	}
+});
diff --git a/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/main.svelte b/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/main.svelte
new file mode 100644
index 0000000000..aedec8c57c
--- /dev/null
+++ b/packages/svelte/tests/runtime-legacy/samples/keyed-each-prod-unique-2/main.svelte
@@ -0,0 +1,8 @@
+<script>
+	let data = [1, 2, 3];
+</script>
+
+<button onclick={() => data = [1, 1, 1]}>add</button>
+{#each data as d (d)}
+	{d}
+{/each}