if error_code=40508
 	then 
 		message('CAN NOT DUPLICATE THIS VALUE');
 		message('CAN NOT DUPLICATE THIS VALUE'); 	
else
	raise form_trigger_failure ;
end if ;

SQL> select upper('Michel Cadot') upper,
  2         lower('Michel Cadot') lower
  3  from dual
  4  /
UPPER        LOWER
------------ ------------
MICHEL CADOT michel cadot

1 row selected.

if error_code=40509
 	then 
 		message('CAN NOT DUPLICATE THIS VALUE');
 		message('CAN NOT DUPLICATE THIS VALUE'); 	
else
	raise form_trigger_failure ;
end if ;

IF :system.trigger_item = 'BLOCK_NAME.PK_COLUMN_NAME' AND 
   error_code=40509
THEN
   MESSAGE('your message here');
   RAISE form_trigger_failure;
END IF;