diff --git a/taskflow/engines/action_engine/analyzer.py b/taskflow/engines/action_engine/analyzer.py
index 1c0f4545..c0b625b6 100644
--- a/taskflow/engines/action_engine/analyzer.py
+++ b/taskflow/engines/action_engine/analyzer.py
@@ -16,6 +16,7 @@
 
 import abc
 import itertools
+import operator
 import weakref
 
 import six
@@ -133,8 +134,9 @@ class Analyzer(object):
     def iter_next_atoms(self, atom=None):
         """Iterate next atoms to run (originating from atom or all atoms)."""
         if atom is None:
-            return iter_utils.unique_seen(self.browse_atoms_for_execute(),
-                                          self.browse_atoms_for_revert())
+            return iter_utils.unique_seen((self.browse_atoms_for_execute(),
+                                           self.browse_atoms_for_revert()),
+                                          seen_selector=operator.itemgetter(0))
         state = self._storage.get_atom_state(atom.name)
         intention = self._storage.get_atom_intention(atom.name)
         if state == st.SUCCESS:
diff --git a/taskflow/engines/action_engine/builder.py b/taskflow/engines/action_engine/builder.py
index 67b333df..72aa81c5 100644
--- a/taskflow/engines/action_engine/builder.py
+++ b/taskflow/engines/action_engine/builder.py
@@ -137,8 +137,8 @@ class MachineBuilder(object):
             # attempt, which may be empty if never ran before) and any nodes
             # that are now ready to be ran.
             memory.next_up.update(
-                iter_utils.unique_seen(self._completer.resume(),
-                                       iter_next_atoms()))
+                iter_utils.unique_seen((self._completer.resume(),
+                                        iter_next_atoms())))
             return SCHEDULE
 
         def game_over(old_state, new_state, event):
diff --git a/taskflow/tests/unit/test_utils_iter_utils.py b/taskflow/tests/unit/test_utils_iter_utils.py
index ad522a4e..4a5ff4b9 100644
--- a/taskflow/tests/unit/test_utils_iter_utils.py
+++ b/taskflow/tests/unit/test_utils_iter_utils.py
@@ -39,9 +39,10 @@ class IterUtilsTest(test.TestCase):
             ['a', 'b'],
             2,
             None,
+            object(),
         ]
         self.assertRaises(ValueError,
-                          iter_utils.unique_seen, *iters)
+                          iter_utils.unique_seen, iters)
 
     def test_generate_delays(self):
         it = iter_utils.generate_delays(1, 60)
@@ -77,7 +78,22 @@ class IterUtilsTest(test.TestCase):
             ['f', 'm', 'n'],
         ]
         self.assertEqual(['a', 'b', 'c', 'd', 'e', 'f', 'm', 'n'],
-                         list(iter_utils.unique_seen(*iters)))
+                         list(iter_utils.unique_seen(iters)))
+
+    def test_unique_seen_empty(self):
+        iters = []
+        self.assertEqual([], list(iter_utils.unique_seen(iters)))
+
+    def test_unique_seen_selector(self):
+        iters = [
+            [(1, 'a'), (1, 'a')],
+            [(2, 'b')],
+            [(3, 'c')],
+            [(1, 'a'), (3, 'c')],
+        ]
+        it = iter_utils.unique_seen(iters,
+                                    seen_selector=lambda value: value[0])
+        self.assertEqual([(1, 'a'), (2, 'b'), (3, 'c')], list(it))
 
     def test_bad_fill(self):
         self.assertRaises(ValueError, iter_utils.fill, 2, 2)
diff --git a/taskflow/utils/iter_utils.py b/taskflow/utils/iter_utils.py
index 413b36f4..959f6660 100644
--- a/taskflow/utils/iter_utils.py
+++ b/taskflow/utils/iter_utils.py
@@ -91,7 +91,7 @@ def generate_delays(delay, max_delay, multiplier=2):
     return _gen_it()
 
 
-def unique_seen(it, *its):
+def unique_seen(its, seen_selector=None):
     """Yields unique values from iterator(s) (and retains order)."""
 
     def _gen_it(all_its):
@@ -99,16 +99,17 @@ def unique_seen(it, *its):
         # can happen before generation/iteration... (instead of
         # during generation/iteration)
         seen = set()
-        while all_its:
-            it = all_its.popleft()
+        for it in all_its:
             for value in it:
-                if value not in seen:
+                if seen_selector is not None:
+                    maybe_seen_value = seen_selector(value)
+                else:
+                    maybe_seen_value = value
+                if maybe_seen_value not in seen:
                     yield value
-                    seen.add(value)
+                    seen.add(maybe_seen_value)
 
-    all_its = collections.deque([it])
-    if its:
-        all_its.extend(its)
+    all_its = list(its)
     for it in all_its:
         if not isinstance(it, collections.Iterable):
             raise ValueError("Iterable expected, but '%s' is"