Constraints - Cannot Enter Multiple Lines Of Data?

I have some tables (below), but I'm having trouble entering multiple lines of data.Here are the tables I have created.

Quote: CUSTOMER table
SQL> create table customer
2 (customer_no char(6) not null,
3 name varchar2(30) not null,
4 address varchar2(50) not null,
5 credit_limit number(6,2),
6 constraint customer_customer_no_pk primary key (customer_no));

Table created.
SALESMAN table
SQL> create table salesman
2 (salesman_id char(8),
3 name varchar2(20) not null,
4 address varchar2(50) not null,
5 emaill_address varchar2(30),
6 constraint salesman_salesman_id_pk primary key (salesman_id));

Table created.
ITEM table
SQL> create table item
2 (ISBN char(13) not null,
3 title varchar2(30) not null,
4 price number(4,2) not null,
5 constraint item_ISBN_pk primary key (ISBN));

Table created.
INVOICE table
SQL> create table invoice
2 (invoice_no char(1) not null,
3 invoice_date date not null,
4 salesman_id char(8),

[code]...

Table created.
DELIVERY table
SQL> create table delivery
2 (invoice_no char(1) not null,
3 ISBN char(13) not null,

[code]...

Table created. I can enter data into the customer, salesman and item table without any problems. However, I need to enter multiple lines of data using the same invoice_no (which is 1). The data is generally the same, except for 2 things need changing but I keep getting the following error...

Quote: ORA-00001: unique constraint(SYSTEM.INVOICE_INVOICE_NO_PK) violated

I understand that I can only use invoice_no once since it is a unique key.