diff --git a/jedi/common.py b/jedi/common.py
index cf45dc88..fc3151af 100644
--- a/jedi/common.py
+++ b/jedi/common.py
@@ -157,4 +157,4 @@ def splitlines(string):
 
 def unite(iterable):
     """Turns a two dimensional array into a one dimensional."""
-    return list(chain.from_iterable(iterable))
+    return set(chain.from_iterable(iterable))
diff --git a/jedi/evaluate/__init__.py b/jedi/evaluate/__init__.py
index bf8fbe3f..6c65ddf8 100644
--- a/jedi/evaluate/__init__.py
+++ b/jedi/evaluate/__init__.py
@@ -187,7 +187,7 @@ class Evaluator(object):
         if isinstance(element, iterable.AlreadyEvaluated):
             return set(element)
         elif isinstance(element, iterable.MergedNodes):
-            return set(iterable.unite(self.eval_element(e) for e in element))
+            return iterable.unite(self.eval_element(e) for e in element)
 
         if_stmt = element.get_parent_until((tree.IfStmt, tree.ForStmt, tree.IsScope))
         predefined_if_name_dict = self.predefined_if_name_dict_dict.get(if_stmt)
diff --git a/jedi/evaluate/helpers.py b/jedi/evaluate/helpers.py
index 91958659..fd33e5c1 100644
--- a/jedi/evaluate/helpers.py
+++ b/jedi/evaluate/helpers.py
@@ -2,7 +2,6 @@ import copy
 from itertools import chain
 
 from jedi.parser import tree
-from jedi import common
 
 
 def deep_ast_copy(obj, parent=None, new_elements=None):
@@ -121,7 +120,7 @@ def get_names_of_node(node):
         else:
             return []
     else:
-        return common.unite(get_names_of_node(c) for c in children)
+        return list(chain.from_iterable(get_names_of_node(c) for c in children))
 
 
 def get_module_names(module, all_scopes):
diff --git a/jedi/evaluate/iterable.py b/jedi/evaluate/iterable.py
index fe419fdf..c31a4897 100644
--- a/jedi/evaluate/iterable.py
+++ b/jedi/evaluate/iterable.py
@@ -265,7 +265,7 @@ class Array(IterableWrapper, ArrayMixin):
 
     @memoize_default()
     def values(self):
-        result = set(unite(self._evaluator.eval_element(v) for v in self._values()))
+        result = unite(self._evaluator.eval_element(v) for v in self._values())
         result |= check_array_additions(self._evaluator, self)
         return result
 
@@ -400,7 +400,7 @@ class FakeDict(_FakeArray):
         self._dct = dct
 
     def get_exact_index_types(self, index):
-        return set(unite(self._evaluator.eval_element(v) for v in self._dct[index]))
+        return unite(self._evaluator.eval_element(v) for v in self._dct[index])
 
     def _items(self):
         return self._dct.items()
@@ -415,7 +415,7 @@ class MergedArray(_FakeArray):
         raise IndexError
 
     def values(self):
-        return set(unite((a.values() for a in self._arrays)))
+        return unite((a.values() for a in self._arrays))
 
     def __iter__(self):
         for array in self._arrays:
@@ -562,10 +562,10 @@ def _check_array_additions(evaluator, compare_array, module, is_list):
             params = params[1:]
         if add_name in ['append', 'add', 'insert']:
             for key, nodes in params:
-                result |= set(unite(evaluator.eval_element(node) for node in nodes))
+                result |= unite(evaluator.eval_element(node) for node in nodes)
         elif add_name in ['extend', 'update']:
             for key, nodes in params:
-                result |= set(unite(get_iterator_types(evaluator, node) for node in nodes))
+                result |= unite(get_iterator_types(evaluator, node) for node in nodes)
         return result
 
     from jedi.evaluate import representation as er, param
diff --git a/jedi/evaluate/stdlib.py b/jedi/evaluate/stdlib.py
index 5988ca23..c5de9ea7 100644
--- a/jedi/evaluate/stdlib.py
+++ b/jedi/evaluate/stdlib.py
@@ -10,6 +10,7 @@ import collections
 import re
 
 from jedi._compatibility import unicode
+from jedi.common import unite
 from jedi.evaluate import compiled
 from jedi.evaluate import representation as er
 from jedi.evaluate import iterable
@@ -49,9 +50,9 @@ def _follow_param(evaluator, arguments, index):
     try:
         key, values = list(arguments.unpack())[index]
     except IndexError:
-        return []
+        return set()
     else:
-        return iterable.unite(evaluator.eval_element(v) for v in values)
+        return unite(evaluator.eval_element(v) for v in values)
 
 
 def argument_clinic(string, want_obj=False, want_scope=False, want_arguments=False):
@@ -152,8 +153,7 @@ def builtins_super(evaluator, types, objects, scope):
 
 def get_iterable_content(evaluator, arguments, argument_index):
     nodes = list(arguments.unpack())[argument_index][1]
-    return set(iterable.unite(iterable.get_iterator_types(evaluator, node)
-                              for node in nodes))
+    return unite(iterable.get_iterator_types(evaluator, node) for node in nodes)
 
 
 @argument_clinic('sequence, /', want_obj=True, want_arguments=True)
@@ -216,8 +216,9 @@ def collections_namedtuple(evaluator, obj, arguments):
         return set()
 
     # Process arguments
-    name = _follow_param(evaluator, arguments, 0)[0].obj
-    _fields = _follow_param(evaluator, arguments, 1)[0]
+    # TODO here we only use one of the types, we should use all.
+    name = list(_follow_param(evaluator, arguments, 0))[0].obj
+    _fields = list(_follow_param(evaluator, arguments, 1))[0]
     if isinstance(_fields, compiled.CompiledObject):
         fields = _fields.obj.replace(',', ' ').split()
     elif isinstance(_fields, iterable.Array):