I have annotated my JSL as to what is the correct JSL to do what you are looking to do
names default to here(1);
dt = Open( "$SAMPLE_DATA/Big Class.jmp" );
// The sort is not required for the Select duplicate rows to work
// The 2 sorts you specified, first sorted all of the rows by age and
// then sorted all of the rows by height. I think what you were looking
// to do, was to sort all of the heights within each age group. That
// would be accomplished with the line of code below
dt << Sort( replace table, by( Column( 2 ),Column( 4 ) ), Order( Ascending, Ascending ) );
dt << Select duplicate rows( Match( Column( dt, 2 ), Column( dt, 3 ), Column( dt, 4 ) ) );
aa = dt <<get selected rows;
If( N Rows( aa ) > 0,
// All you need is a simple assignment statement setting all of the rows found for the column
// age equal to 9999
(Column(dt,2)[aa]) = 9999;
);
Jim