Answers for "ON DUPLICATE KEY"

mysql on duplicate key update

INSERT INTO t1 (a,b,c) VALUES (1,2,3)
  ON DUPLICATE KEY UPDATE c=3;
INSERT INTO t1 (a,b,c) VALUES (4,5,6)
  ON DUPLICATE KEY UPDATE c=9;

sql insert or update

-- Oracle: Example for Insert or update in t1 from t2 values 
MERGE INTO table1 t1
USING table2 t2
ON (t1.CODE = t2.ID)
WHEN MATCHED THEN
    UPDATE SET t1.COL1 = t2.VALUE1
WHEN NOT MATCHED THEN
    INSERT (CODE, COL1)  VALUES (t2.ID, t2.VALUE1);

-- MySql (makes a INSERT + DELETE if existing)
REPLACE INTO table1 (pk_id, col1) VALUES (5, 'aaaa');
--
INSERT INTO table1 VALUES (key, generation)
ON DUPLICATE KEY UPDATE (key = key, generation = generation + 1);

duplicate entry for key

The error says it all:

Duplicate entry '' 
So run the following query:

SELECT unique_id,COUNT(unique_id)
FROM yourtblname
GROUP BY unique_id
HAVING COUNT(unique_id) >1
This query will also show you the problem

SELECT *
FROM yourtblname
WHERE unique_id=''
This will show you where there are values that have duplicates. You are trying to create a unique index on a field with duplicates. You will need to resolve the duplicate data first then add the index