precommit
diff --git a/‎pre_commit/repository.py‎
Lines changed: 12 additions & 1 deletion b/‎pre_commit/repository.py‎
Lines changed: 12 additions & 1 deletion
diff --git a/‎tests/repository_test.py‎
Lines changed: 16 additions & 2 deletions b/‎tests/repository_test.py‎
Lines changed: 16 additions & 2 deletions
@@ -64,7 +64,7 @@ def languages(self):
 
     @cached_property
     def additional_dependencies(self):
-        dep_dict = defaultdict(lambda: defaultdict(set))
+        dep_dict = defaultdict(lambda: defaultdict(_UniqueList))
         for _, hook in self.hooks:
             dep_dict[hook['language']][hook['language_version']].update(
                 hook.get('additional_dependencies', []),
@@ -222,3 +222,14 @@ def sha(self):
     @cached_property
     def manifest(self):
         raise NotImplementedError
+
+
+class _UniqueList(list):
+    def __init__(self):
+        self._set = set()
+
+    def update(self, obj):
+        for item in obj:
+            if item not in self._set:
+                self._set.add(item)
+                self.append(item)
@@ -477,7 +477,20 @@ def test_additional_dependencies(tempdir_factory, store):
     config = make_config_from_repo(path)
     config['hooks'][0]['additional_dependencies'] = ['pep8']
     repo = Repository.create(config, store)
-    assert repo.additional_dependencies['python']['default'] == {'pep8'}
+    assert repo.additional_dependencies['python']['default'] == ['pep8']
+
+
+@pytest.mark.integration
+def test_additional_dependencies_duplicated(
+        tempdir_factory, store, log_warning_mock,
+):
+    path = make_repo(tempdir_factory, 'ruby_hooks_repo')
+    config = make_config_from_repo(path)
+    config['hooks'][0]['additional_dependencies'] = [
+        'thread_safe', 'tins', 'thread_safe']
+    repo = Repository.create(config, store)
+    assert repo.additional_dependencies['ruby']['default'] == [
+        'thread_safe', 'tins']
 
 
 @pytest.mark.integration
@@ -517,12 +530,13 @@ def test_additional_ruby_dependencies_installed(
 ):  # pragma: no cover (non-windows)
     path = make_repo(tempdir_factory, 'ruby_hooks_repo')
     config = make_config_from_repo(path)
-    config['hooks'][0]['additional_dependencies'] = ['thread_safe']
+    config['hooks'][0]['additional_dependencies'] = ['thread_safe', 'tins']
     repo = Repository.create(config, store)
     repo.require_installed()
     with ruby.in_env(repo.cmd_runner, 'default'):
         output = cmd_output('gem', 'list', '--local')[1]
         assert 'thread_safe' in output
+        assert 'tins' in output
 
 
 @skipif_slowtests_false