[feaLib] Reject duplicate languagesystem declarations

Lib/fontTools/feaLib/builder.py

@@ -145,6 +145,10 @@ class Builder(object):
             raise FeatureLibError(
                 'If "languagesystem DFLT dflt" is present, it must be '
                 'the first of the languagesystem statements', location)
+        if (script, language) in self.default_language_systems_:
+            raise FeatureLibError(
+                '"languagesystem %s %s" has already been specified' %
+                (script.strip(), language.strip()), location)
         self.default_language_systems_.add((script, language))
 
     def get_default_language_systems_(self):

Lib/fontTools/feaLib/builder_test.py

@@ -124,6 +124,12 @@ class BuilderTest(unittest.TestCase):
         self.assertEqual(builder.language_systems,
                          {('latn', 'FRA'), ('cyrl', 'RUS')})
 
+    def test_languagesystem_duplicate(self):
+        self.assertRaisesRegex(
+            FeatureLibError,
+            '"languagesystem cyrl RUS" has already been specified',
+            self.build, "languagesystem cyrl RUS; languagesystem cyrl RUS;")
+
     def test_languagesystem_none_specified(self):
         builder = Builder(None, TTFont())
         builder.start_feature(location=None, name='test')