warnings: adding new filter resets prior existing "once" filters

Bug report

Bug description:

If a filter is defined with action="once", the occurrence tracking is reset each time a new filter is defined. This can override the ability to effectively use action="once" if the filter manipulation occurs in a library which the user cannot control (e.g. matplotlib).

For example, the following code will only output a warning once (i.e. works as intended):

import warnings

warnings.filterwarnings("once", "test warning", UserWarning)
warnings.warn("test warning", UserWarning)
warnings.warn("test warning", UserWarning)

However, simply inserting an unrelated filter prior to the second warning will cause both to be output:

warnings.filterwarnings("once", "test warning", UserWarning)
warnings.warn("test warning", UserWarning)

warnings.filterwarnings("ignore", "something unrelated")
warnings.warn("test warning", UserWarning)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-153697

Inizia con il reproducer di warnings.filterwarnings e warnings.warn presente nell’issue, quindi individua l’implementazione di warnings e i relativi test in CPython. Il lavoro è completo quando l’aggiunta di un filtro non correlato non reimposta più il tracciamento delle occorrenze per un filtro esistente con action="once"; gh-153697 è già collegato a questa issue.