Emit the duplicate key as part of the error message when validating keyed each blocks

src/runtime/internal/keyed_each.ts

@@ -112,7 +112,7 @@ export function validate_each_keys(ctx, list, get_context, get_key) {
 	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');
+			throw new Error(`Cannot have duplicate keys in a keyed each: ${key} is a duplicate`);
 		}
 		keys.add(key);
 	}

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'
+	error: 'Cannot have duplicate keys in a keyed each: 1 is a duplicate'
 };