Bug report

Checklist

• I am confident this is a bug in CPython,
  not a bug in a third-party project
• I have searched the CPython issue tracker,
  and am confident this bug has not been reported before

A clear and concise description of the bug

Currently, we do not have much control over the behavior of ConfigParser when it encounters a duplicate section header or a duplicate option inside a section. It will directly raise an exception in strict mode and the read will fail. In case of non-strict mode, it will parse the config but there's no way to add custom behavior to it.
For example, I had a business requirement where we had to log a warning incase a duplicate section/option is encountered. But to do this I had to subclass the ConfigParser and override the entire _read method with a lot of code duplication just to add two log lines.
Another use case would be having different duplicate handling for Sections and Options.

MRE:

Consider this config file:

[Section1]
opt1 = 1
opt2 = 2

[Section2]
opt1 = 1
opt2 = 2
opt1 = 3

[Section1]
opt1 = 2

When I do ConfigParser().read("config.ini"), this will raise a DuplicateOptionError and the code will stop executing. If I pass strict=False, it will parse as expected but there is no way to intimate to the end user that the config contains duplicates.

Your environment

• CPython versions tested on: 3.9, 3.10, 3.11
• Operating system and architecture: Windows x64

Linked PRs

• gh-107428: Added overridable methods for handling duplicate sections and options in ConfigParser. #107430