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

先從 issue 中的 warnings.filterwarnings 和 warnings.warn 重現案例開始，然後在 CPython 中找到 warnings 的實作及其測試。完成的標準是：加入無關的篩選器後，不再重設現有 action="once" 篩選器的出現次數追蹤；gh-153697 已經連結到此 issue。