Describe the issue:

If the following code is run under Python 3.13.1t, it fails nondeterministically with A cast was already added for <class 'numpy.dtype[rational]'> -> <class 'numpy.dtypes.Int8DType'>. (method: legacy_cast).

Reproduce the code example:

import concurrent.futures
import functools
import threading
import numpy as np
import numpy._core._rational_tests as _rational_tests

num_threads = 1000

def closure(b):
    b.wait()
    for _ in range(100):
        np.full((10, 10), 1, _rational_tests.rational)


with concurrent.futures.ThreadPoolExecutor(max_workers=num_threads) as executor:
    b = threading.Barrier(num_threads)
    futures = [executor.submit(functools.partial(closure, b)) for _ in range(num_threads)]
    [f.result() for f in futures]

Error message:

Traceback (most recent call last):
  File "/Users/goldbaum/.pyenv/versions/3.13.1t/lib/python3.13t/site-packages/numpy/_core/numeric.py", line 353, in full
    multiarray.copyto(a, fill_value, casting='unsafe')
RuntimeError: A cast was already added for <class 'numpy.dtype[rational]'> -> <class 'numpy.dtypes.Int8DType'>. (method: legacy_cast)

Python and NumPy Versions:

2.3.0.dev0+git20241219.35b2c4a
3.13.1 experimental free-threading build (tags/v3.13.1:06714517797, Dec 15 2024, 15:38:01) [Clang 18.1.8 (11)]

Runtime Environment:

[{'numpy_version': '2.3.0.dev0+git20241219.35b2c4a',
'python': '3.13.1 experimental free-threading build '
'(tags/v3.13.1:06714517797, Dec 15 2024, 15:38:01) [Clang 18.1.8 '
'(11)]',
'uname': uname_result(system='Linux', node='', release='', version='https://github.com/https://github.com//pull/1 SMP PREEMPT_DYNAMIC Debian 6.redacted (2024-10-16)', machine='x86_64')},
{'simd_extensions': {'baseline': ['SSE', 'SSE2', 'SSE3'],
'found': ['SSSE3',
'SSE41',
'POPCNT',
'SSE42',
'AVX',
'F16C',
'FMA3',
'AVX2'],
'not_found': ['AVX512F',
'AVX512CD',
'AVX512_KNL',
'AVX512_SKX',
'AVX512_CLX',
'AVX512_CNL',
'AVX512_ICL']}},
{'architecture': 'Zen',
'filepath': '/usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.27.so',
'internal_api': 'openblas',
'num_threads': 128,
'prefix': 'libopenblas',
'threading_layer': 'pthreads',
'user_api': 'blas',
'version': '0.3.27'}]

Context for the issue:

Found when working on free-threading support in JAX.