Duplicates in different config sources are treated differently

[ajcvickers]

See dotnet/extensions#600

Specifically, duplicates within a given source. For example, the same setting repeated twice in an .ini file.

These providers throw either ArgumentException or FormatException:

• AzureKeyVault
• Ini
• KeyPerFile
• XML
• Memory

These providers ignore duplicates if the case does match:

• Json

These providers ignore duplicates if the case does not match:

• EnvironmentVariables

These providers ignore duplicates:

• CommandLine

[ajcvickers]

See:

• Issue #689: Raise Exception for Duplicate Key aspnet/Configuration#782
• MemoryConfigurationProvider.cs to handle duplicate keys (should use last one added) aspnet/Configuration#835

[ajcvickers]

Notes from triage:

• Providers that currently ignore duplicates should continue to do so to avoid breaking existing apps
• For 3.0 we would consider a change that consolidated all the exception types/messages across providers into one consistent exception and message. (This is technically also a breaking change, but one that is unlikely to cause real-world issues. Still; better to do it in 3.0 than in a non-breaking release.)

[maryamariyan]

This issue seems to have a dupe. I will link once I find it.

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