duplicate columns with COPY

Is there a reason why COPY TO STDOUT does not allow columns to be specified 
more than once?

pei=# copy test1 (a, a) to stdout;
ERROR:  42701: column "a" specified more than once

Or is this just an overly extensive check that is actually intended for COPY 
FROM STDIN?

-- 
Peter Eisentraut
http://developer.postgresql.org/~petere/