create primary key on top of nonunique index

cat pk.on.nonunique.index.sql
drop table nonunique purge;
create table nonunique ( i number );
create index pk_i on nonunique ( i );
alter table nonunique add constraint pk_i primary key ( i );
@ sel.pkdemo.md.sql NONUNIQUE

SQL> @ pk.on.nonunique.index.sql

Table dropped.


Table created.


Index created.


Table altered.


TABLE_NAME                     COLUMN_NAME                    NUL
------------------------------ ------------------------------ ---
NONUNIQUE                      I                              N


TABLE_NAME                     INDEX_NAME                     UNIQUENESS                  STATUS
------------------------------ ------------------------------ --------------------------- ------------------------
NONUNIQUE                      PK_I                           NONUNIQUE                   VALID


TABLE_NAME                     INDEX_NAME                     COLUMN_NAME
------------------------------ ------------------------------ ------------------------------
NONUNIQUE                      PK_I                           I


CONSTRAINT_NAME                CON STATUS                   VALIDATED
------------------------------ --- ------------------------ ---------------------------------------
PK_I                           P   ENABLED                  VALIDATED

You can create a primary key constraint on top of a nonunique index.