add test

test/runtime/samples/keyed-each-dev-unique/_config.js

@@ -0,0 +1,7 @@
+export default {
+	compileOptions: {
+		dev: true
+	},
+
+	error: `Cannot have duplicate keys in a keyed each`
+};

test/runtime/samples/keyed-each-dev-unique/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	const array = [1, 2, 3, 1];
+</script>
+
+{#each array as item (item)}
+	{item}
+{/each}