Caveat on duplicated indices

Author: crusaderky

array_api_compat/common/_helpers.py

@@ -886,6 +886,13 @@ class at:
     In the above example, y == [2, 0, 0] and z == [0, 3, 0] when x is read-only,
     whereas y == z == [2, 3, 0] when x is writeable!
 
+    Caveat
+    ------
+    The behaviour of methods other than `get()` when the index is an array of
+    integers which contains multiple occurrences of the same index is undefined.
+
+    **Undefined behaviour:** ``at(x, [0, 0]).set(2)``
+
     See Also
     --------
     https://jax.readthedocs.io/en/latest/_autosummary/jax.numpy.ndarray.at.html