Combine and Internalize Three Queries in MySQL

Hi all - this may not be possible:

We'd like to run a query like the one below to find duplicate entries in our recordset, save those records to a separate table, and update the source table with a flag to mark the record as found.

Currently, we're doing all 3 outside access and one at a time:

Query 1) SELECT *
FROM MyTable AS mt
WHERE (((Exists (SELECT ColumnA, ColumnB, ColumnC
                         FROM MyTable AS a
                         WHERE  a.ColumnA = mt.ColumnA  
                         AND  ABS(a.ColumnB) = ABS(mt.ColumnB)  
                         AND  a.ColumnC = mt.Columnc    
                         GROUP BY a.ColumnA, a.ColumnB, a.ColumnC
                         HAVING Count(*)>1))<>False))
ORDER BY mt.ColumnA, mt.ColumnB, mt.ColumnC;

Query 2) loop through all records returned from Query 1 and insert those records in a new table, but process Query 3 below before moving on to next record from Query 1. There will be some blank/null values for ColumnG so I'm not sure what if/then language is available/needed.
INSERT INTO dup1b_report (
        MyTableID,
        ColumnA,
        ColumnB,
        ColumnC,
        ColumnD,
        ColumnE,
        ColumnF
        <if Len(ColumnG) GTE 1>, ColumnG</if>)
VALUES             (
        QUERY1.ID Value,
        QUERY1.ColumnA Value,
        QUERY1.ColumnB Value,
        QUERY1.ColumnC Value,
       ' QUERY1.ColumnD TextValue',
        QUERY1.ColumnE DateValue,
        QUERY1.ColumnF Value
        <if Len(ColumnG) GTE 1>, ColumnG Value</if>)

Query 3) Record that this loops record was identified in the original source table, then go on to the next record returned by Query 1.
UPDATE       MyTable
SET       MyTable.dup       =       1
WHERE      MyTable.ID      =      Query1.ID Value

Can MySQL do this all by itself? If so how? Can it handle running on large record sets? (10's and 100's of millions of rows) We also have a full copy of SQL Server 2005 standard if that is a better platform for handling this type of issue but we REALLY don't know that software.

Thanks for your help!