Different casing in variables causes exception

Case

Add the same configuration key value pair to from different configuration sources using different casing.

Expected result

Configuration value from last configuration source is the value present in the final configuration.

The documentation states that

Configuration keys are not case-sensitive

and

Configuration sources are read in the order that they’re specified. In the preceding code, the environment variables are read last. Any configuration values set through the environment replace those set in the two previous providers.

Actual result

ArgumentException with the message An item with the same key has already been added is thrown.

Sample program

var configurationRoot = new ConfigurationBuilder()
    .AddInMemoryCollection(new Dictionary<string, string>
        {
            {"CASE:PROBLEM", "black magic"}
        })
    .AddInMemoryCollection(new Dictionary<string, string>
        {
            {"case:PROBLEM", "white magic"}
        })
    .Build();

var newconfigurationRoot = new ConfigurationBuilder()
    .AddInMemoryCollection(configurationRoot.AsEnumerable())
    .Build();

[rasmus]

We didn’t recover from this. Until this is fixed we educate our developers to make sure to use lower casing for all settings 😢

Use case: We pull settings from multiple sources, e.g. Consul, environment variables, appsettings.json, command line, Vault etc. in order to give both developers and site operations maximum flexibility. Keeping casing of settings across all these sources was a pain until we decided to merely use lowercase for everything.

[ajcvickers]

Fixed by https://github.com/aspnet/Extensions/pull/503