I am not certain if this is true in all recent versions of JMP. I make this comment only to highlight an opportunity to improve the robustness of the code.
'Contains' returns the position of the item, which is fine to use in an 'if' statement, since zero will be interpreted as false and a positive value will be interpreted true. When preceding a contains with a not '!', the condition works when the item is not present or when present as the first item; however, when the item isn't first ( a value greater than 1 ), the not '!' can fail to convert the condition to false. This can be avoided by using a comparison '>0' with the 'contains'. See below for syntax.
dt = Open( "$sample_data\Big Class.jmp" );
col_name_list = dt << get column names( string );
new_column = "BMI";
// English BMI Formula
// BMI = ( Weight in Pounds / ( Height in inches x Height in inches ) ) x 703
If( !(Contains( col_name_list, new_column ) > 0),
dt << New Column( "BMI", numeric, continuous, formula( 703 * :weight / :height / :height ) )
);