Build failed with shared mbed-os using Windows junction

[ccli8]

Describe the bug
On Windows, when mbed-os is shared using WIndows junction, build failed with:

Configuring project and generating build system...
Traceback (most recent call last):
  File "c:\python38\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\python38\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Python38\Scripts\mbed-tools.exe\__main__.py", line 7, in <module>
  File "c:\python38\lib\site-packages\click\core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "c:\python38\lib\site-packages\click\core.py", line 782, in main
    rv = self.invoke(ctx)
  File "c:\python38\lib\site-packages\mbed_tools\cli\main.py", line 38, in invoke
    super().invoke(context)
  File "c:\python38\lib\site-packages\click\core.py", line 1259, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "c:\python38\lib\site-packages\click\core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "c:\python38\lib\site-packages\click\core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "c:\python38\lib\site-packages\mbed_tools\cli\build.py", line 105, in build
    config, _ = generate_config(mbed_target.upper(), toolchain, program)
  File "c:\python38\lib\site-packages\mbed_tools\build\config.py", line 36, in generate_config
    config = assemble_config(
  File "c:\python38\lib\site-packages\mbed_tools\build\_internal\config\assemble_build_config.py", line 42, in assemble_config
    return _assemble_config_from_sources(target_attributes, mbed_lib_files, mbed_app_file)
  File "c:\python38\lib\site-packages\mbed_tools\build\_internal\config\assemble_build_config.py", line 64, in _assemble_config_from_sources
    config.update(source.from_file(config_file, target_filters=current_filter_data.labels))
  File "c:\python38\lib\_collections_abc.py", line 832, in update
    self[key] = other[key]
  File "c:\python38\lib\site-packages\mbed_tools\build\_internal\config\config.py", line 27, in __setitem__
    self._update_config_section(item)
  File "c:\python38\lib\site-packages\mbed_tools\build\_internal\config\config.py", line 66, in _update_config_section
    raise ValueError(
ValueError: Setting fat_chan.ffs_dbg already defined. You cannot duplicate config settings!

To Reproduce

1. On Windows, clone mbed-os-example-blinky and mbed-os to different directories:

   $ git clone https://github.com/ARMmbed/mbed-os-example-blinky -b mbed-os-6.15.0
   $ git clone https://github.com/ARMmbed/mbed-os -b mbed-os-6.15.0
   

2. Under mbed-os-example-blinky, run mklink /J to link mbed-os to just downloaded path
3. Under mbed-os-example-blinky, build with:

   mbed-tools -vvv compile -m <TARGET> -t <TOOLCHAIN>
   

Expected behavior
Build OK

Screenshots
N/A

Desktop (please complete the following information):

• OS: Windows
• Version: 10

Mbed (please complete the following information):

• Device: All
• Mbed OS Version: 6.15.0
• Mbed CLI 2 Version: 7.44.0

If Mbed CLI 2 changes back to 7.16.0, build OK.

Additional context
N/A

[0xc0170]

Isn't this valid error in the config : Setting fat_chan.ffs_dbg already defined. You cannot duplicate config settings! ? Are there two duplicated settings in Mbed OS ? I haven't seen this error in CI, it could be we have older cli 2 there?

[ccli8]

Some points to re-produce the error:

1. mbed-os needs to be shared with Windows junction mklink /J
2. mbed cli2 needs to be newer version: 7.16.0 OK and 7.44.0 failed