How find out …

Find duplicate rows
SELECT A.field FROM YOURTABLENAME A
WHERE rowid > (SELECT min(rowid) FROM YOURTABLENAME B WHERE B.field = A.field );

remove duplicate rows:
DELETE FROM YOURTABLENAME
WHERE rowid in
(SELECT rowid FROM
(SELECT rowid, row_number() over (partition by key order by key) dup FROM YOURTABLENAME)
WHERE dup > 1);

or

DELETE FROM YOURTABLENAME
WHERE rowid not in (SELECT MIN(rowid)
FROM YOURTABLENAME
GROUP BY column1, column2, column3…);

Tablespace of a table
SELECT YOURTABLENAME FROM all_tables
WHERE table_name = ‘YOURTABLENAME’;