Make sure to call each invariant only once when validating invariants.

.gitignore

@@ -2,6 +2,7 @@
 *.pyc
 *.pyo
 *.so
+*.swp
 __pycache__
 .coverage
 .coverage.*

CHANGES.rst

@@ -12,6 +12,9 @@
   that argument. See `issue 208
   <https://github.com/zopefoundation/zope.interface/issues/208>`_.
 
+- When an invariant is defined in an interface, it's found by
+  `validateInvariants` in all interfaces inheriting from that interface.
+  Make sure to call each invariant only once when validating invariants.
 
 5.1.0 (2020-04-08)
 ==================

src/zope/interface/interface.py

@@ -875,9 +875,14 @@ def direct(self, name):
     def queryDescriptionFor(self, name, default=None):
         return self.get(name, default)
 
-    def validateInvariants(self, obj, errors=None):
+    def validateInvariants(self, obj, errors=None, seen=None):
         """validate object to defined invariants."""
+        if seen is None:
+            seen = set()
         for call in self.queryTaggedValue('invariants', []):
+            if call in seen:
+                continue
+            seen.add(call)
             try:
                 call(obj)
             except Invalid as e:
@@ -886,7 +891,7 @@ def validateInvariants(self, obj, errors=None):
                 errors.append(e)
         for base in self.__bases__:
             try:
-                base.validateInvariants(obj, errors)
+                base.validateInvariants(obj, errors, seen=seen)
             except Invalid:
                 if errors is None:
                     raise

src/zope/interface/tests/test_interface.py

@@ -1014,6 +1014,20 @@ def _fail(*args, **kw):
         self.assertEqual(len(_errors), 1)
         self.assertTrue(isinstance(_errors[0], Invalid))
 
+    def test_validateInvariants_inherited_not_called_multiple_times(self):
+        _passable_called_with = []
+
+        def _passable(*args, **kw):
+            _passable_called_with.append((args, kw))
+            return True
+
+        obj = object()
+        base = self._makeOne('IBase')
+        base.setTaggedValue('invariants', [_passable])
+        derived = self._makeOne('IDerived', (base,))
+        derived.validateInvariants(obj)
+        self.assertEqual(1, len(_passable_called_with))
+
     def test___reduce__(self):
         iface = self._makeOne('PickleMe')
         self.assertEqual(iface.__reduce__(), 'PickleMe')