[racket] Function redefinition and "define-values: cannot re-define a constant:"

  Hi

Select mode is "Determinate language from source"
Run the next code in the source buffer

#lang scheme
(define (foo x) (+ x x))

Then go to REPL and type

 > (define (foo x) (- x x))

it will returned with message: define-values: cannot re-define a 
constant: foo

It does not happen if mode is "Pretty Big" or "R5RS" (this case #lang 
should be eliminated)

Why that happen?