Description
Currently it is neither an error for diagnostic structs to have fields of the same name as subdiagnostics used in that same struct, nor is it a runtime error to specify arg("foo", ...) twice with the same arg name. This caused some confusion in const eval errors where the main error had an instance field/arg and the stack frame that got printed had an instance per frame
Originally posted by @RalfJung in #142015 (comment)