I checked why our tests did not find this, and it's because they generate random 32-bit values which are very unlikely to have many duplicates. We can also run them with 8-bit values.

@Mousius could you add a test in NumPy to add coverage for such sorting array with duplicate values?
Ref #27037 (comment)

@r-devulap lets not talk about how the test was the first thing I wrote and still forgot to include it in the PR 🙀

Thanks @Mousius .