The current documentation for ToImmutableDictionary does not document that it throws on a key collision.

https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutabledictionary.toimmutabledictionary?view=net-9.0

When converting a collection from a to b, the intuitive behavior would be to either skip the value or offer a method overload that gives the user an escape hatch. Since, it doesn't appear an escape hatch exists from the current api, I think at minimum this should be documented on the extension methods.