better error msg for duplicate definitions

[pnwamk]

This program

#lang pie

(claim foo Atom)
(define foo 'f)

(define foo TODO)

Currently errors with the error message "No claim: foo ". It seems like it would be much more intuitive to say something like "foo is already defined" or similar.

I'm happy to look into this but don't want to context switch at the moment -- pushing it onto the proverbial stack ;-)