dup field in sub-class should be reported by the compiler

[Walter Bright]

Brad Roberts wrote:
> On Mon, 13 Aug 2007, Walter Bright wrote:
>> I just don't see the issue.
> 
> Apply this same logic to variable shadowing.  Shadowing has been declared 
> illegal.

While pedantically it is the same logic, but practically I think it's 
very different. I don't see writing derived classes like one writes a 
nested scope.