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Drew Adams writes:

 > Shouldn't substitute-command-keys remove duplicates?
 >  
 > If you explicitly bind a key to the same command in both a
 > keymap km
 > and its parent pkm, and then you call this, the binding is
 > written
 > twice:
 >  
 > (define-key km  "a" 'foo)
 > (define-key pkm "a" 'foo)
 >  
 > (substitute-command-keys "\\{km\\}")
 >  
 > key             binding
 > ---             -------
 >  
 > ...
 > a  foo
 > a  foo