[BUG]: Misleading errors on duplicate enum key registration

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What version (or hash if on master) of pybind11 are you using?

3.0.1

Problem description

I added duplicate enum keys by accident:

   py::native_enum<StandardIconType>(m, "StandardIconType", "enum.IntEnum")
        .value("TitleBarMenuButton", TitleBarMenuButton)
        .value("TitleBarMenuButton", TitleBarMenuButton)
        …

Now i get lots of warnings like:

15:36:31 [warn:albert.python] ImportError: initialization failed

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/color/__init__.py(17): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

15:36:31 [debg:albert] Plugin python.color unloaded. ImportError: initialization failed

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/color/__init__.py(17): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

15:36:32 [warn:albert.python] ImportError: generic_type: type "PluginInstance" is already registered!

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/firefox/__init__.py(9): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

15:36:32 [debg:albert] Plugin python.firefox unloaded. ImportError: generic_type: type "PluginInstance" is already registered!

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/firefox/__init__.py(9): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

15:36:32 [warn:albert.python] ImportError: generic_type: type "MatchConfig" is already registered!

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/jetbrains_projects/__init__.py(43): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

15:36:32 [debg:albert] Plugin python.jetbrains_projects unloaded. ImportError: generic_type: type "MatchConfig" is already registered!

At:
  /Users/manuel/Library/Application Support/albert/python/plugins/jetbrains_projects/__init__.py(43): <module>
  <frozen importlib._bootstrap>(488): _call_with_frames_removed
  <frozen importlib._bootstrap_external>(1026): exec_module

Basically for each import of the embedded module I get weird is already registered! errros for types that are not part of the actual problem. It took me quite a while to find the actual problem statement (the duplicate enum above).

I would be useful if the errord could indicate the actually duplicate registration of the enum key instead of random other symbols.

Reproducible example code

See above

Is this a regression? Put the last known working version here if it is.

Not a regression

Start by reproducing the duplicate .value("TitleBarMenuButton", ...) registration in py::native_enum and trace the registration path that reports the later generic_type errors. Check how duplicate enum keys are validated and how the resulting exception propagates during embedded-module imports. Done means the reported error identifies the duplicate enum key rather than unrelated already-registered types.