BUG: Fix is_unique regression for slices of Indexes

doc/source/whatsnew/v2.2.2.rst

@@ -15,6 +15,7 @@ Fixed regressions
 ~~~~~~~~~~~~~~~~~
 - :meth:`DataFrame.__dataframe__` was producing incorrect data buffers when the a column's type was a pandas nullable on with missing values (:issue:`56702`)
 - :meth:`DataFrame.__dataframe__` was producing incorrect data buffers when the a column's type was a pyarrow nullable on with missing values (:issue:`57664`)
+- :meth:`Index.is_unique` could incorrectly return false if the ``Index`` was created from a slice of another ``Index``. (:issue:`57911`)
 -
 
 .. ---------------------------------------------------------------------------

pandas/_libs/index.pyx

@@ -253,8 +253,10 @@ cdef class IndexEngine:
         return self.sizeof()
 
     cpdef _update_from_sliced(self, IndexEngine other, reverse: bool):
-        self.unique = other.unique
-        self.need_unique_check = other.need_unique_check
+        if other.unique:
+            self.unique = 1
+            self.need_unique_check = 0
+
         if not other.need_monotonic_check and (
                 other.is_monotonic_increasing or other.is_monotonic_decreasing):
             self.need_monotonic_check = other.need_monotonic_check
@@ -854,8 +856,10 @@ cdef class SharedEngine:
         pass
 
     cpdef _update_from_sliced(self, ExtensionEngine other, reverse: bool):
-        self.unique = other.unique
-        self.need_unique_check = other.need_unique_check
+        if other.unique:
+            self.unique = 1
+            self.need_unique_check = 0
+
         if not other.need_monotonic_check and (
                 other.is_monotonic_increasing or other.is_monotonic_decreasing):
             self.need_monotonic_check = other.need_monotonic_check

pandas/tests/indexes/test_base.py

@@ -963,6 +963,13 @@ def test_slice_keep_name(self):
         index = Index(["a", "b"], name="asdf")
         assert index.name == index[1:].name
 
+    def test_slice_is_unique(self):
+        # GH 57911
+        index = Index([1, 1, 2, 3, 4])
+        assert not index.is_unique
+        filtered_index = index[2:].copy()
+        assert filtered_index.is_unique
+
     @pytest.mark.parametrize(
         "index",
         [