Fix error in windows provider when the environment has accidental duplicates that differ only by case.
Make the provider storage for the environment on windows ignore duplicates and only report the effective value.
Add tests to verify existing environment get-item behavior and to ensure that Get-Item env:<var> reports the same as $env:<var>, namely the effective value.
Fixes#6305 and supersedes #6320 based on discussion in #6460.
87834d07b4