Add B033: Duplicate items in sets

README.rst

@@ -186,6 +186,8 @@ second usage. Save the result to a list if the result is needed multiple times.
 
 **B032**: Possible unintentional type annotation (using ``:``). Did you mean to assign (using ``=``)?
 
+**B033**: Sets should not contain duplicate items. Duplicate items will be replaced with a single item at runtime.
+
 Opinionated warnings
 ~~~~~~~~~~~~~~~~~~~~
 
@@ -329,6 +331,7 @@ Unreleased
 ~~~~~~~~~~
 
 * B030: Fix crash on certain unusual except handlers (e.g. ``except a[0].b:``)
+* Add B033: Check for duplicate items in sets.
 
 23.3.12
 ~~~~~~~~

bugbear.py

@@ -518,6 +518,10 @@ def visit_Import(self, node):
         self.check_for_b005(node)
         self.generic_visit(node)
 
+    def visit_Set(self, node):
+        self.check_for_b033(node)
+        self.generic_visit(node)
+
     def check_for_b005(self, node):
         if isinstance(node, ast.Import):
             for name in node.names:
@@ -1308,6 +1312,14 @@ def check_for_b032(self, node):
         ):
             self.errors.append(B032(node.lineno, node.col_offset))
 
+    def check_for_b033(self, node):
+        constants = [
+            item.value
+            for item in filter(lambda x: isinstance(x, ast.Constant), node.elts)
+        ]
+        if len(constants) != len(set(constants)):
+            self.errors.append(B033(node.lineno, node.col_offset))
+
 
 def compose_call_path(node):
     if isinstance(node, ast.Attribute):
@@ -1705,6 +1717,13 @@ def visit_Lambda(self, node):
     )
 )
 
+B033 = Error(
+    message=(
+        "B033 Sets should not contain duplicate items. Duplicate items will be replaced"
+        " with a single item at runtime."
+    )
+)
+
 # Warnings disabled by default.
 B901 = Error(
     message=(

tests/b033.py

@@ -0,0 +1,17 @@
+"""
+Should emit:
+B033 - on lines 6-12
+"""
+
+test = {1, 2, 3, 3, 5}
+test = {"a", "b", "c", "c", "e"}
+test = {True, False, True}
+test = {None, True, None}
+test = {3, 3.0}
+test = {1, True}
+test = {0, False}
+
+test = {1, 2, 3, 3.5, 5}
+test = {"a", "b", "c", "d", "e"}
+test = {True, False}
+test = {None}

tests/test_bugbear.py

@@ -44,6 +44,7 @@
     B030,
     B031,
     B032,
+    B033,
     B901,
     B902,
     B903,
@@ -489,6 +490,21 @@ def test_b032(self):
         )
         self.assertEqual(errors, expected)
 
+    def test_b033(self):
+        filename = Path(__file__).absolute().parent / "b033.py"
+        bbc = BugBearChecker(filename=str(filename))
+        errors = list(bbc.run())
+        expected = self.errors(
+            B033(6, 7),
+            B033(7, 7),
+            B033(8, 7),
+            B033(9, 7),
+            B033(10, 7),
+            B033(11, 7),
+            B033(12, 7),
+        )
+        self.assertEqual(errors, expected)
+
     @unittest.skipIf(sys.version_info < (3, 8), "not implemented for <3.8")
     def test_b907(self):
         filename = Path(__file__).absolute().parent / "b907.py"