Preventing duplicates in an SQL query?

Set the column as unique.

More on this

http://php.about.com/od/mysqlcommands/g/add_unique.htm

As has been said by hgulyan, if you want a table.col1 to be unique, add a unique constraint on the column.

If what you mean is that you want the insert to fail if column table1.col1 = table1.col2, then you can implement this in a trigger. See MYSQL Create Trigger.

To raise an exception in the trigger, so that the insert fails, see TRIGGERs that cause INSERTs to fail? Possible?

Something like:

CREATE TRIGGER Employee_beforeinsert before insert
ON Employee FOR EACH ROW
    BEGIN
    IF new.age = new.age2 THEN
        DECLARE dummy INT;

        SELECT 'Your meaningful error message goes here' INTO dummy 
        FROM Employee
        WHERE Employee.id=new.id
    END IF;
END;

The following is Standard SQL, rather than mySQL dialect, but mySQL is has a good level of compliance with the Standard and I trust you should be able to follow my point.

You didn't post your schema or sample data so I had to guess what your tables might look like:

CREATE TABLE Mothers
(
 mother_ID INTEGER NOT NULL UNIQUE
);

CREATE TABLE Children
(
 child_ID INTEGER NOT NULL UNIQUE
);

CREATE TABLE MothersOfTwins
(
 mother_ID INTEGER NOT NULL 
    UNIQUE REFERENCES Mothers (mother_ID), 
 twin_1_child_ID INTEGER 
    REFERENCES Children (child_ID), 
 twin_2_child_ID INTEGER 
    REFERENCES Children (child_ID), 
 CHECK (twin_1_child_ID <> twin_2_child_ID)
);

INSERT INTO Mothers (mother_ID) VALUES (101), (102), (103);

INSERT INTO Children (child_ID) VALUES (551), (552), (553), (554);

INSERT INTO MothersOfTwins (mother_ID, twin_1_child_ID, twin_2_child_ID) 
VALUES 
(101, 551, 552), 
(102, 552, 551);  -- duplicate

That last INSERT succeeds even though it should fail i.e. transposing the child_ID values between rows will fool any UNIQUE constraint you care to put on the columns. I guess this is similar to the problem you are facing.

One solution to this problem is to create a base table which requires multiple rows to model siblings, using an 'occurrence' column with a constraint to ensure there cannot be more than two siblings (i.e. twins):

DROP TABLE MothersOfTwins;

CREATE TABLE MothersOfTwinsBase
(
 mother_ID INTEGER NOT NULL
    REFERENCES Mothers (mother_ID), 
 twin_occurrence INTEGER NOT NULL
    CHECK (twin_occurrence BETWEEN 1 AND 2), 
 twin_child_ID INTEGER NOT NULL UNIQUE 
    REFERENCES Children (child_ID), 
 UNIQUE (mother_ID, twin_occurrence)
);

INSERT INTO MothersOfTwinsBase (mother_ID, twin_occurrence, twin_child_ID) 
VALUES 
(101, 1, 551), 
(101, 2, 552),
(102, 1, 553),
(103, 2, 554);

You can then recreate the data structure of your former base table using a VIEW e.g.

CREATE VIEW MothersOfTwins
(
 mother_ID, 
 twin_1_child_ID, twin_2_child_ID
)
AS
SELECT M1.mother_ID, 
       M1.twin_child_ID AS twin_1_child_ID, 
       M2.twin_child_ID AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M1 
       INNER JOIN MothersOfTwinsBase AS M2
          ON M1.mother_ID = M2.mother_ID
             AND M1.twin_occurrence = 1
             AND M2.twin_occurrence = 2
UNION ALL
SELECT M1.mother_ID, 
       M1.twin_child_ID AS twin_1_child_ID, 
       NULL AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M1 
 WHERE NOT EXISTS (
                   SELECT *
                     FROM MothersOfTwinsBase AS M2
                          WHERE M1.mother_ID = M2.mother_ID
                                AND M2.twin_occurrence = 2
                  )
UNION ALL
SELECT M2.mother_ID, 
       NULL AS twin_1_child_ID, 
       M2.twin_child_ID AS twin_2_child_ID
  FROM MothersOfTwinsBase AS M2
 WHERE NOT EXISTS (
                   SELECT *
                     FROM MothersOfTwinsBase AS M1
                          WHERE M1.mother_ID = M2.mother_ID
                                AND M1.twin_occurrence = 1
                  );