Squashed commit of the following:

commit b151e80
Author: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Date:   Sat Feb 27 15:46:18 2021 +0100

    Use flag to guard DuplicateBasesError

commit 3bbf4d2
Author: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Date:   Tue Feb 23 01:48:29 2021 +0100

    Add test cases for duplicate bases error

commit 934264d
Author: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Date:   Mon Feb 22 11:00:48 2021 +0100

    Fix duplicate bases error for typing._GenericAlias

    * Fixes pylint-dev#905

ChangeLog

@@ -11,6 +11,10 @@ Release Date: TBA
 
   Closes #895 #899
 
+* Use flag to guard DuplicateBasesError
+
+  Closes #905
+
 What's New in astroid 2.5?
 ============================
 Release Date: 2021-02-15

astroid/scoped_nodes.py

@@ -99,13 +99,20 @@ def _c3_merge(sequences, cls, context):
     return None
 
 
-def clean_duplicates_mro(sequences, cls, context):
+def clean_duplicates_mro(
+    sequences, cls, context, raise_duplicate_bases_error: bool = True
+):
     for sequence in sequences:
         names = [
             (node.lineno, node.qname()) if node.name else None for node in sequence
         ]
         last_index = dict(map(reversed, enumerate(names)))
-        if names and names[0] is not None and last_index[names[0]] != 0:
+        if (
+            raise_duplicate_bases_error is True
+            and names
+            and names[0] is not None
+            and last_index[names[0]] != 0
+        ):
             raise exceptions.DuplicateBasesError(
                 message="Duplicates found in MROs {mros} for {cls!r}.",
                 mros=sequences,
@@ -2816,7 +2823,7 @@ def slots(self):
 
         def grouped_slots():
             # Not interested in object, since it can't have slots.
-            for cls in self.mro()[:-1]:
+            for cls in self.mro(raise_duplicate_bases_error=False)[:-1]:
                 try:
                     cls_slots = cls._slots()
                 except NotImplementedError:
@@ -2871,15 +2878,18 @@ def _inferred_bases(self, context=None):
             else:
                 yield from baseobj.bases
 
-    def _compute_mro(self, context=None):
+    def _compute_mro(self, context=None, raise_duplicate_bases_error: bool = True):
         inferred_bases = list(self._inferred_bases(context=context))
         bases_mro = []
         for base in inferred_bases:
             if base is self:
                 continue
 
             try:
-                mro = base._compute_mro(context=context)
+                mro = base._compute_mro(
+                    context=context,
+                    raise_duplicate_bases_error=raise_duplicate_bases_error,
+                )
                 bases_mro.append(mro)
             except NotImplementedError:
                 # Some classes have in their ancestors both newstyle and
@@ -2891,18 +2901,26 @@ def _compute_mro(self, context=None):
                 bases_mro.append(ancestors)
 
         unmerged_mro = [[self]] + bases_mro + [inferred_bases]
-        unmerged_mro = list(clean_duplicates_mro(unmerged_mro, self, context))
+        unmerged_mro = list(
+            clean_duplicates_mro(
+                unmerged_mro, self, context, raise_duplicate_bases_error
+            )
+        )
         return _c3_merge(unmerged_mro, self, context)
 
-    def mro(self, context=None) -> List["ClassDef"]:
+    def mro(
+        self, context=None, raise_duplicate_bases_error: bool = True
+    ) -> List["ClassDef"]:
         """Get the method resolution order, using C3 linearization.
 
         :returns: The list of ancestors, sorted by the mro.
         :rtype: list(NodeNG)
         :raises DuplicateBasesError: Duplicate bases in the same class base
         :raises InconsistentMroError: A class' MRO is inconsistent
         """
-        return self._compute_mro(context=context)
+        return self._compute_mro(
+            context=context, raise_duplicate_bases_error=raise_duplicate_bases_error
+        )
 
     def bool_value(self, context=None):
         """Determine the boolean value of this node.

tests/unittest_scoped_nodes.py

@@ -1270,8 +1270,18 @@ class NodeBase(object):
         assert len(slots) == 3, slots
         assert [slot.value for slot in slots] == ["a", "b", "c"]
 
-    def assertEqualMro(self, klass, expected_mro):
-        self.assertEqual([member.name for member in klass.mro()], expected_mro)
+    def assertEqualMro(
+        self, klass, expected_mro, raise_duplicate_bases_error: bool = True
+    ):
+        self.assertEqual(
+            [
+                member.name
+                for member in klass.mro(
+                    raise_duplicate_bases_error=raise_duplicate_bases_error
+                )
+            ],
+            expected_mro,
+        )
 
     @unittest.skipUnless(HAS_SIX, "These tests require the six library")
     def test_with_metaclass_mro(self):
@@ -1436,6 +1446,45 @@ class C(scope.A, scope.B):
         )
         self.assertEqualMro(cls, ["C", "A", "B", "object"])
 
+    @test_utils.require_version("3.7", "3.9")
+    def test_mro_with_duplicate_generic_alias(self):
+        """Catch false positive. Assert no error is thrown."""
+        cls = builder.extract_node(
+            """
+        from typing import Sized, Hashable
+        class Derived(Sized, Hashable):
+            def __init__(self):
+                self.var = 1
+        """
+        )
+        self.assertEqualMro(
+            cls,
+            ["Derived", "_GenericAlias", "_Final", "object"],
+            raise_duplicate_bases_error=False,
+        )
+
+    @test_utils.require_version("3.9")
+    def test_mro_with_duplicate_generic_alias_2(self):
+        cls = builder.extract_node(
+            """
+        from typing import Sized, Hashable
+        class Derived(Sized, Hashable):
+            def __init__(self):
+                self.var = 1
+        """
+        )
+        self.assertEqualMro(
+            cls,
+            [
+                "Derived",
+                "_SpecialGenericAlias",
+                "_BaseGenericAlias",
+                "_Final",
+                "object",
+            ],
+            raise_duplicate_bases_error=False,
+        )
+
     def test_generator_from_infer_call_result_parent(self):
         func = builder.extract_node(
             """