SQL & PL/SQL

I have a table with the following structure in Oracle database:

CREATE TABLE PASSENGERS
*(ID VARCHAR2(6),*
PASSPORTNO VARCHAR2(14));

I want to get the IDs of the passengers who have been registered more than once. For that I run the following query.

SELECT ID FROM PASSENGERS WHERE PASSPORTNO IN
*(SELECT PASSPORTNO FROM PASSENGERS*
GROUP BY PASSPORTNO
HAVING COUNT()>1);*

But I get "unsuported character set" error. If I try to run the following queries separately I don't get any error:

SELECT PASSPORTNO FROM PASSENGERS
GROUP BY PASSPORTNO
HAVING COUNT()>1;*


SELECT COUNT(PASSPORTNO) FRO PASSENGERS;

SELECT ID FROM PASSENGERS;

What's the point I'm missing?