Single Table Hash Clusters

I created a single table hash cluster like this :

create tablespace mssm datafile 'c:appmssm01.dbf' size 100m
segment space management manual;

create cluster hash_cluster_4k
( id number(2) )
size 8192 single table hash is id hashkeys 4 tablespace mssm;

-- Created a table in cluster with row size such that only one record fits one block and inserted 5 records each with a distinct key value

CREATE TABLE hash_cluster_tab_8k
( id number(2) ,
txt1 char(2000),
txt2 char(2000),
txt3 char(2000)
)
CLUSTER hash_cluster_8k( id );
[code]....

If I issue the same query after creating unique index on hash_cluster_tab(id), the execution plan shows hash access and single I/O (cr = 1).Does it mean that to have single I/o in a single table hash cluster, we have to create unique index? Won't it create additional overhead of maintaining an index?

What is the second I/O needed for in case unique index is absent?