Auto merge of #12318 - tallowen:tidy-features, r=frewsxcv

[tidy] prevent duplicate json keys

Throws a tidy error when duplicated keys are present.
![screenshot from 2016-07-07 16-54-17](fd19cece-4465-11e6-89b1-a79296ba6a72.png)

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12283 (github issue number if applicable).

<!-- Either: -->
- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/12318)
<!-- Reviewable:end -->

python/tidy/servo_tidy/tidy.py

@@ -524,16 +524,28 @@ def check_webidl_spec(file_name, contents):
     yield (0, "No specification link found.")
 
 
+def check_for_possible_duplicate_json_keys(key_value_pairs):
+    keys = [x[0] for x in key_value_pairs]
+    seen_keys = set()
+    for key in keys:
+        if key in seen_keys:
+            raise KeyError(key)
+
+        seen_keys.add(key)
+
+
 def check_json(filename, contents):
     if not filename.endswith(".json"):
         raise StopIteration
 
     try:
-        json.loads(contents)
+        json.loads(contents, object_pairs_hook=check_for_possible_duplicate_json_keys)
     except ValueError as e:
         match = re.search(r"line (\d+) ", e.message)
         line_no = match and match.group(1)
         yield (line_no, e.message)
+    except KeyError as e:
+        yield (None, "Duplicated Key (%s)" % e.message)
 
 
 def check_spec(file_name, lines):

python/tidy/servo_tidy_tests/duplicate_key.json

@@ -0,0 +1,7 @@
+{
+    "key": "value",
+    "other_key": {
+        "the_duplicated_key": 1,
+        "the_duplicated_key": 2
+    }
+}

python/tidy/servo_tidy_tests/test_tidy.py

@@ -109,6 +109,11 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('Invalid control character at: line 3 column 40 (char 61)', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
+    def test_json_with_duplicate_key(self):
+        errors = tidy.collect_errors_for_files(iterFile('duplicate_key.json'), [tidy.check_json], [], print_text=False)
+        self.assertEqual('Duplicated Key (the_duplicated_key)', errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
     def test_lock(self):
         errors = tidy.collect_errors_for_files(iterFile('duplicated_package.lock'), [tidy.check_lock], [], print_text=False)
         msg = """duplicate versions for package "test"