use index instead

src/runtime/internal/keyed_each.ts

@@ -108,12 +108,12 @@ export function update_keyed_each(old_blocks, dirty, get_key, dynamic, ctx, list
 }
 
 export function validate_each_keys(ctx, list, get_context, get_key) {
-	const keys = new Set();
+	const keys = new Map();
 	for (let i = 0; i < list.length; i++) {
 		const key = get_key(get_context(ctx, list, i));
 		if (keys.has(key)) {
-			throw new Error(`Cannot have duplicate keys in a keyed each: ${key} is a duplicate`);
+			throw new Error(`Cannot have duplicate keys in a keyed each: Keys at index ${keys.get(key)} and ${i} are duplicates`);
 		}
-		keys.add(key);
+		keys.add(key, i);
 	}
 }

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

@@ -3,5 +3,5 @@ export default {
 		dev: true
 	},
 
-	error: 'Cannot have duplicate keys in a keyed each: 1 is a duplicate'
+	error: 'Cannot have duplicate keys in a keyed each: Keys at index 0 and 3 are duplicates'
 };