diff --git a/sc2/units.py b/sc2/units.py
index 3e58b1d2..7431db13 100644
--- a/sc2/units.py
+++ b/sc2/units.py
@@ -50,11 +50,9 @@ def __or__(self, other: Units) -> Units:
         """
         :param other:
         """
+        self_tags = {self_unit.tag for self_unit in self}
         return Units(
-            chain(
-                iter(self),
-                (other_unit for other_unit in other if other_unit.tag not in (self_unit.tag for self_unit in self)),
-            ),
+            chain(iter(self), (other_unit for other_unit in other if other_unit.tag not in self_tags)),
             self._bot_object,
         )
 
@@ -62,11 +60,9 @@ def __add__(self, other: Units) -> Units:
         """
         :param other:
         """
+        self_tags = {self_unit.tag for self_unit in self}
         return Units(
-            chain(
-                iter(self),
-                (other_unit for other_unit in other if other_unit.tag not in (self_unit.tag for self_unit in self)),
-            ),
+            chain(iter(self), (other_unit for other_unit in other if other_unit.tag not in self_tags)),
             self._bot_object,
         )
 
@@ -74,8 +70,9 @@ def __and__(self, other: Units) -> Units:
         """
         :param other:
         """
+        self_tags = {self_unit.tag for self_unit in self}
         return Units(
-            (other_unit for other_unit in other if other_unit.tag in (self_unit.tag for self_unit in self)),
+            (other_unit for other_unit in other if other_unit.tag in self_tags),
             self._bot_object,
         )
 
@@ -83,8 +80,9 @@ def __sub__(self, other: Units) -> Units:
         """
         :param other:
         """
+        other_tags = {other_unit.tag for other_unit in other}
         return Units(
-            (self_unit for self_unit in self if self_unit.tag not in (other_unit.tag for other_unit in other)),
+            (self_unit for self_unit in self if self_unit.tag not in other_tags),
             self._bot_object,
         )
 
diff --git a/test/test_pickled_data.py b/test/test_pickled_data.py
index fac77b71..a867f09f 100644
--- a/test/test_pickled_data.py
+++ b/test/test_pickled_data.py
@@ -804,6 +804,14 @@ def test_units():
     assert scvs.random_or(1)
     assert townhalls.random_or(0)
     assert scvs.random_group_of(11)
+    # Set operations dedupe by tag and keep the order: self first, then other
+    head = Units(scvs[:6], bot)
+    tail = Units(scvs[4:], bot)
+    assert [u.tag for u in head | tail] == [u.tag for u in scvs]
+    assert [u.tag for u in head + tail] == [u.tag for u in scvs]
+    assert [u.tag for u in head & tail] == [u.tag for u in scvs[4:6]]
+    assert [u.tag for u in head - tail] == [u.tag for u in scvs[:4]]
+    assert [u.tag for u in head - head] == []
     assert not scvs.random_group_of(0)
     assert not townhalls.random_group_of(0)
     # assert not scvs.in_attack_range_of(townhalls.first)