0. #lang racket
1.  
2. (define fish%
3.   (class object%
4.     (init color)
5.  
6.     (define color color)
7.  
8.     (super-new)
9.  
10.     (define/public (get-color)
11.       color)))

=>

eval:2:0: class: duplicate declared identifier

at: color

in: (class object% (init color) (define color color)

(super-new) (define/public (get-color) color))