lint: disallow duplicate values in "features" tag (#3010)

tools/lint/lib/checks/features.py

@@ -29,3 +29,6 @@ class CheckFeatures(Check):
         for feature in features:
             if feature not in self.valid_features:
                 return 'Unrecognized feature: "%s"' % feature
+
+        if len(set(features)) != len(features):
+            return 'The `features` tag may not include duplicate entries'

tools/lint/test/fixtures/test/features_duplicated.js

@@ -0,0 +1,11 @@
+FEATURES
+^ expected errors | v input
+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-assignment-operators-static-semantics-early-errors
+description: Duplicated values in "features" frontmatter
+features: [object-spread, async-functions, object-spread]
+---*/
+
+async function f({ ...a }) {}