SQL & PL/SQL :: Primary And Foreign Key On Same Column And Foreign Key Index

Create table Car
(Car_cd VARCHAR2(5),
Car_Desc VARCHAR2(50)
);
alter table Car
add constraint Car_PK primary key (Car_CD);
INSERT INTO Car (Car_Cd, Car_Desc) VALUES ('CORLA','COROLLA');
Commit;
[code]....

The requirement necessitates a new table to map car to manufacturer. This mapping table may later be expanded to contain other attributes Engine, MPG, etc to hold details specific to a car.But this is in future.

Create Table Car_Mapping_Details
(Car_Cd VARCHAR2(5),
Manufacturer_Cd VARCHAR2(5));

--Primary Key Constraint
ALTER TABLE Car_Mapping_Details
ADD CONSTRAINT Car_Mapping_Details_pk PRIMARY KEY (Car_Cd );

--Not able to create this as Car_cd is already a PK in this table and therefore has Unique Index
ALTER TABLE Car_Mapping_Details ADD CONSTRAINT Car_Mapping_Details_fk1
FOREIGN KEY (Car_Cd)REFERENCES Car (Car_Cd);
[code]....

But in this case the Car_Mapping_Details.Car_cd is itself is a primary key and therefore has Unique index.Although I was able to create foreign key constraint on Car_mapping_details.car_cd column (which is also Primary Key), I was not able to create Foreign Key Index on this column. It gives me Quote:ORA-01408: such column list already indexed.In other words, not creating foreign index for foreign key column lead to table-level lock? Or will the Unique Index in that primary key column be sufficient to avoid table-level lock?