What advice can you give me on how to track down a bug I have while inserting data into MySQL database with .Net?

The error message is:

MySql.Data.MySqlClient.MySqlException: Duplicate entry '26012' for key 'StockNumber_Number_UNIQUE'

Reviewing of the log proves that StockNumber_Number of 26012 has not been inserted yet.

Products in use.

• Visual Studio 2008.
• mysql.data.dll 6.0.4.0.
• Windows 7 Ultimate 64 bit and Windows 2003 32 bit.
• Custom built ORM framework (have source code).
• Importing data from Access 2003 database.

The code works fine for 3000 - 5000 imports.

The record being imported that causes the problem in a full run works fine if just importing by itself. I've also seen the error on other records if I sort the data to be imported a different way.

Have tried import with and without transactions.

Have logged the hell out of the system.

The SQL command to create the table:

CREATE TABLE `RareItems_RareItems` (
    `RareItemKey` CHAR(36) NOT NULL PRIMARY KEY, 
    `StockNumber_Text` VARCHAR(7) NOT NULL, 
    `StockNumber_Number` INT NOT NULL AUTO_INCREMENT,  
    UNIQUE INDEX `StockNumber_Number_UNIQUE` (`StockNumber_Number` ASC), 
    `OurPercentage` NUMERIC ,  
    `SellPrice` NUMERIC(19, 2) ,  
    `Author` VARCHAR(250) ,  
    `CatchWord` VARCHAR(250) ,  
    `Title` TEXT ,  
    `Publisher` VARCHAR(250) ,  
    `InternalNote` VARCHAR(250) ,  
    `DateOfPublishing` VARCHAR(250) ,  
    `ExternalNote` LONGTEXT ,  
    `Description` LONGTEXT ,  
    `Scrap` LONGTEXT ,  
    `SuppressionKey` CHAR(36) NOT NULL,  
    `TypeKey` CHAR(36) NOT NULL,  
    `CatalogueStatusKey` CHAR(36) NOT NULL,  
    `CatalogueRevisedDate` DATETIME ,  
    `CatalogueRevisedByKey` CHAR(36) NOT NULL,  
    `CatalogueToBeRevisedByKey` CHAR(36) NOT NULL,  
    `DontInsure` BIT NOT NULL,  
    `ExtraCosts` NUMERIC(19, 2) ,  
    `IsWebReady` BIT NOT NULL,  
    `LocationKey` CHAR(36) NOT NULL,  
    `LanguageKey` CHAR(36) NOT NULL,  
    `CatalogueDescription` VARCHAR(250) ,  
    `PlacePublished` VARCHAR(250) ,  
    `ToDo` LONGTEXT ,  
    `Headline` VARCHAR(250) ,  
    `DepartmentKey` CHAR(36) NOT NULL,  
    `Temp1` INT ,  
    `Temp2` INT ,  
    `Temp3` VARCHAR(250) ,  
    `Temp4` VARCHAR(250) ,  
    `InternetStatusKey` CHAR(36) NOT NULL,  
    `InternetStatusInfo` LONGTEXT ,  
    `PurchaseKey` CHAR(36) NOT NULL,  
    `ConsignmentKey` CHAR(36) ,  
    `IsSold` BIT NOT NULL,  
    `RowCreated` DATETIME NOT NULL,  
    `RowModified` DATETIME NOT NULL
);

The SQL command and parameters to insert the record:

INSERT INTO `RareItems_RareItems` 
    (`RareItemKey`, `StockNumber_Text`, `StockNumber_Number`, `OurPercentage`, `SellPrice`, `Author`, `CatchWord`, `Title`, `Publisher`, `InternalNote`, `DateOfPublishing`, `ExternalNote`, `Description`, `Scrap`, `SuppressionKey`, `TypeKey`, `CatalogueStatusKey`, `CatalogueRevisedDate`, `CatalogueRevisedByKey`, `CatalogueToBeRevisedByKey`, `DontInsure`, `ExtraCosts`, `IsWebReady`, `LocationKey`, `LanguageKey`, `CatalogueDescription`, `PlacePublished`, `ToDo`, `Headline`, `DepartmentKey`, `Temp1`, `Temp2`, `Temp3`, `Temp4`, `InternetStatusKey`, `InternetStatusInfo`, `PurchaseKey`, `ConsignmentKey`, `IsSold`, `RowCreated`, `RowModified`) 
VALUES
    (@RareItemKey, @StockNumber_Text, @StockNumber_Number, @OurPercentage, @SellPrice, @Author, @CatchWord, @Title, @Publisher, @InternalNote, @DateOfPublishing, @ExternalNote, @Description, @Scrap, @SuppressionKey, @TypeKey, @CatalogueStatusKey, @CatalogueRevisedDate, @CatalogueRevisedByKey, @CatalogueToBeRevisedByKey, @DontInsure, @ExtraCosts, @IsWebReady, @LocationKey, @LanguageKey, @CatalogueDescription, @PlacePublished, @ToDo, @Headline, @DepartmentKey, @Temp1, @Temp2, @Temp3, @Temp4, @InternetStatusKey, @InternetStatusInfo, @PurchaseKey, @ConsignmentKey, @IsSold, @RowCreated, @RowModified)

@RareItemKey = 0b625bd6-776d-43d6-9405-e97159d172a6
@StockNumber_Text = 199305
@StockNumber_Number = 26012
@OurPercentage = 22.5
@SellPrice = 1250
@Author = SPARRMAN, Anders.
@CatchWord = COOK: SECOND VOYAGE
@Title = A Voyage Round the World with Captain James Cook in H.M.S. Resolution… Introduction and notes by Owen Rutter, wood engravings by Peter Barker-Mill.
@Publisher = 
@InternalNote = 
@DateOfPublishing = 1944
@ExternalNote = The first English translation of Sparrman’s narrative, which had originally     been published in Sweden in 1802-1818, and the only complete version of his account to appear in English. The eighteenth-century translation had appeared some time before the Swedish publication of the final sections of his account. Sparrman’s observant and well-written narrative of the second voyage contains much that appears nowhere else, emphasising naturally his interests in medicine, health, and natural history.<br><br>One of 350 numbered copies: a handsomely produced and beautifully illustrated work.
@Description = Small folio, wood-engravings in the text; original olive glazed cloth, top edges gilt, a very good copy. London, Golden Cockerel Press, 1944.
@Scrap = 
@SuppressionKey = 00000000-0000-0000-0000-000000000000
@TypeKey = 93f58155-7471-46ad-84c5-262ab9dd37e8
@CatalogueStatusKey = 00000000-0000-0000-0000-000000000003
@CatalogueRevisedDate = 
@CatalogueRevisedByKey = c4f6fc06-956d-44c4-b393-0d5462cbffec
@CatalogueToBeRevisedByKey = 00000000-0000-0000-0000-000000000000
@DontInsure = False
@ExtraCosts = 
@IsWebReady = False
@LocationKey = 00000000-0000-0000-0000-000000000000
@LanguageKey = 00000000-0000-0000-0000-000000000000
@CatalogueDescription = 
@PlacePublished = Golden Cockerel Press
@ToDo = 
@Headline = 
@DepartmentKey = 529578a3-9189-40de-b656-eef9039d00b8
@Temp1 = 
@Temp2 = 
@Temp3 = 
@Temp4 = v
@InternetStatusKey = 00000000-0000-0000-0000-000000000000
@InternetStatusInfo = 
@PurchaseKey = 00000000-0000-0000-0000-000000000000
@ConsignmentKey = 
@IsSold = True
@RowCreated = 8/04/2010 8:49:16 PM
@RowModified = 8/04/2010 8:49:16 PM

Suggestions on what is causing the error and/or how to track down what is causing the problem?