Actionable diagnostic for duplicated directives

[Gedochao]

Is your feature request related to a problem? Please describe. Duplicated using directives don't cause any warnings. Declaring a dependency twice with different versions will cause one of them to be ignored.

Describe the solution you'd like Detect duplication of using directives in project and suggest users to have only one. For example in the project we should have only one using directive to specify scala version //> using scala. An actionable diagnostic would be useful.

Additional context extracted from https://github.com/VirtusLab/scala-cli/issues/1542