SQL>create table tbl(n number primary key)
SQL> /

Table created.

SQL> create unique index on tbl(N);
create unique index on tbl(N)
                    *
ERROR at line 1:
ORA-00953: missing or invalid index name


SQL> ed
Wrote file afiedt.buf

  1* create unique index indx on tbl(N)
SQL> /
create unique index indx on tbl(N)
                                *
ERROR at line 1:
ORA-01408: such column list already indexed

Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options

orcl10g SCOTT>create table foo (foo varchar2(80));

Table created.

orcl10g SCOTT>select * from user_indexes where table_name='FOO';

no rows selected

orcl10g SCOTT>create index foox on foo(foo);

Index created.

orcl10g SCOTT>select * from user_indexes where table_name='FOO';

INDEX_NAME INDEX_TY TABLE_ TABLE_NAME           TABLE_TYPE  UNIQUENES
---------- -------- ------ -------------------- ----------- ---------
FOOX       NORMAL   SCOTT  FOO                  TABLE       NONUNIQUE

orcl10g SCOTT>alter table foo add constraint foop primary key (foo);

Table altered.

orcl10g SCOTT>select * from user_indexes where table_name='FOO';

INDEX_NAME INDEX_TY TABLE_ TABLE_NAME           TABLE_TYPE  UNIQUENES
---------- -------- ------ -------------------- ----------- ---------
FOOX       NORMAL   SCOTT  FOO                  TABLE       NONUNIQUE

orcl10g SCOTT>select CONSTRAINT_NAME,constraint_type,TABLE_NAME,INDEX_NAME from user_constraints where table_name='FOO';

CONSTRAINT_NAME  C TABLE_NAME   INDEX_NAME
---------------- - ------------ ------------
FOOP             P FOO          FOOX

13:39:21 SQL> CREATE TABLE ID10T (id number, col1 varchar2(12));

Table created.

13:39:21 SQL> INSERT INTO ID10T VALUES (1,'REDUNDANCY');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (2,'IS');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (3,'BEST');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (4,'WAY');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (5,'TO');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (6,'TEACH');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (7,'SOME');

1 row created.

13:39:21 SQL> INSERT INTO ID10T VALUES (8,'FOLKS');

1 row created.

13:39:21 SQL> CREATE INDEX NOT_UNIQ_IDX ON ID10T (ID);

Index created.

13:39:21 SQL> ALTER TABLE ID10T ADD PRIMARY KEY (ID);

Table altered.

13:39:21 SQL> INSERT INTO ID10T VALUES (8,'!');
INSERT INTO ID10T VALUES (8,'!')
*
ERROR at line 1:
ORA-00001: unique constraint (BCM.SYS_C0018449) violated

13:42:23 SQL> select constraint_name, column_name from user_cons_columns where table_name = 'ID10T';

CONSTRAINT_NAME                                                                            COLUMN_NAME
------------------------------------------------------------------------------------------ --------------------------------
SYS_C0018449                                                                               ID

13:43:14 SQL> select constraint_type from user_constraints where constraint_name = 'SYS_C0018449';

CON
---
P

13:45:51 SQL> SELECT INDEX_NAME, INDEX_TYPE FROM USER_INDEXES WHERE TABLE_NAME = 'ID10T';

INDEX_NAME       INDEX_TYPE
-------------------------------
NOT_UNIQ_IDX     NORMAL

Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production
With the Partitioning, OLAP and Data Mining options

SQL> CREATE TABLE foo (x NUMBER CONSTRAINT x_id PRIMARY KEY DEFERRABLE, y VARCHAR2(20));

Table created.

SQL> SELECT index_name, uniqueness
  2  FROM user_indexes
  3  WHERE table_name = 'FOO';

INDEX_NAME                     UNIQUENES
------------------------------ ---------
X_ID                           NONUNIQUE