Why does parallel for add duplicate rows when I change the second number and how can I stop this from happening?

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VB

Thanks to Richard I was able to make it one step further and incorporated his additional coding into my current project.

Public Sub Match_Merge(I As Long)

Is the method I am call for the parallel for.

inside this method I am comparing my temp table which is my import table and by current database table.

For I = 1 To temptable.Rows.Count - 1

                    countI = countI + 1
                    'this will be table 2

                    For I2 = I3 To dt.Rows.Count - 1
Next I
Next i2

if there is not a match then it the information goes to

removeduptable.ImportRow(temptable.Rows(I))

if there a match then the information is stored in a separate table.

duplicaterow = Duplicatetable.NewRow()
                            duplicaterow.Item("First_name") = dt.Rows(I2).Item("First_name")
                            duplicaterow.Item("Middle_name") = dt.Rows(I2).Item("Middle_name")
                            duplicaterow.Item("Last_name") = dt.Rows(I2).Item("Last_name")
                            duplicaterow.Item("Duplicatecount") = count

                            'add rows to table
                            Duplicatetable.Rows.Add(duplicaterow)

This is the method I am using for my parallel for:

Public Sub ParallelForEach(ByVal startOfIteration As Integer, ByVal endOfIteration As Integer, ByVal subFunctionName As String)

        processorcount.MaxDegreeOfParallelism = 4
        Const flags As BindingFlags = BindingFlags.Public Or BindingFlags.NonPublic Or BindingFlags.Static Or BindingFlags.Instance

        ' Find the method with the specified name which accepts a single Integer parameter:
        Dim method As MethodInfo = Me.GetType().GetMethod(subFunctionName, flags, Nothing, New Type() {GetType(Long)}, Nothing)

        If method Is Nothing Then
            Throw New ArgumentException(String.Format("Method '{0}' was not found.", subFunctionName), "subFunctionName")
        End If

        ' Create a delegate of the correct type pointing to the method:
        Dim del As System.Delegate
        If method.IsStatic Then
            del = System.Delegate.CreateDelegate(GetType(Action(Of Long)), method)
        Else
            del = System.Delegate.CreateDelegate(GetType(Action(Of Long)), Me, method)
        End If

        ' Cast the delegate to the correct type:
        Dim action As Action(Of Long) = DirectCast(del, Action(Of Long))

        ' NB: Second parameter to Range is the number of items to return, not the last number to return:
        '      Dim numbers As IEnumerable(Of Long) = Enumerable.Range(startOfIteration, 1 + endOfIteration - startOfIteration)


        System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = False
        System.Threading.Tasks.Parallel.For(1, 2, processorcount, action)
        
    End Sub

If I leave the parallel for like this:

System.Threading.Tasks.Parallel.For(1, 2, processorcount, action)

it runs as fast as a sequential.

if I change it to

System.Threading.Tasks.Parallel.For(1, 10, processorcount, action)

it runs quicker, but the parallel for adds in duplicate rows.

So if the record count is 150,000 it will show up as 300,000 I can not figure out a way to stop this from happening.

What I have tried:

I tried to cut off the row add when the count made it to the amount of temptable rows. I also tried to synclock both tables and that did not help either.