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

Issue にある warnings.filterwarnings と warnings.warn の再現コードから始め、その後 CPython 内の warnings の実装とテストを特定します。既存の action="once" フィルターについて、無関係なフィルターを追加しても発生回数の追跡がリセットされなくなれば完了です。gh-153697 はすでにこの issue にリンクされています。