SQL & PL/SQL :: Rebuild The Table Structure?

--this for txn details
CREATE TABLE txn_det(
txnid NUMBER PRIMARY KEY,
amount NUMBER,
status varchar2(50),
cust_id NUMBER);
----this for customer details
CREATE TABLE cust_det(
cust_id NUMBER PRIMARY KEY,
cust_name VARCHAR2(50),
cust_acc number(15));

--data to insert for customer table
INSERT INTO cust_det VALUES(101,'Miller','12345');

INSERT INTO cust_det VALUES(201,'Scott','45678');
----data to insert for txn table
INSERT INTO txn_det VALUES('tx0045',123.00,'success',101);

INSERT INTO txn_det VALUES('tx0046',4512.50,'success',101);

insert into txn_det values('tx0049',78.12,'success',101);

INSERT INTO txn_det VALUES('tx0055',123.12,'success',201);

Now THE problem IS cust_det TABLE's cust_id coulmn may contain duplicate.So I thought OF adding THE txn_id COLUMN TO THE cust_det table but I know that encourgaes redundancy.