WL#928: Disallow duplicate names in some column lists

It should be an error to use the same column name 
twice in column lists where the column cannot 
actually appear twice. 
 
Example: 
 
CREATE TABLE t (s1 INT, PRIMARY KEY (s1,s1)) 
/* the above is illegal */ 
 
SELECT * FROM x AS y (s1,s1) 
/* the aboe is illegal */