constraint.  This case (a) is useless, (b) violates SQL92, and
(c) is certain to cause a failure downstream when we try to create
an index with duplicated column names.  So give an appropriate error
message instead of letting the index failure occur.  Per report from
Colin Strickland.  NOTE: currently, CREATE INDEX fooi ON foo(f1,f1)
still fails with 'cannot insert duplicate key' error.  Should we
change that too?  What about functional indexes?