timer control ...executing function issue

[Meenu]

Advance thanks to all...

I am using a timer control for calling a function (lastposition()) in each 20 seconds.Now the problem is first time there is no issue.Second time function is executing two times and the third time function is executing 3 times like that it is increasing.I dont understand what logic is missing in my code.

after 20 second lastposition function is executing 1 time
after 40 second lastposition function is executing 2 time
after 60 second lastposition function is executing 3 time
after 80 second lastposition function is executing 4 time.......

Like this ...Can anybdy check my below code and tell me what mistake in this?

Partial Public Class MainPage
    Inherits UserControl
    Dim _timerRealTime As New Storyboard()




Private Sub ChkRealTime_Checked(sender As Object, e As RoutedEventArgs) Handles ChkRealTime.Checked

     _timerRealTime.Duration = TimeSpan.FromMilliseconds(1)

     AddHandler _timerRealTime.Completed, AddressOf Me._timerRealtime_Completed

     _timerRealTime.Begin()

 End Sub

 

 Private Sub _timerRealtime_Completed(sender As Object, e As EventArgs)

     LastPosition()

     _timerRealTime.Duration = TimeSpan.FromSeconds(20)

     _timerRealTime.Begin()

 End Sub

 

 Private Sub LastPosition()

     lastposdate = ""

     DoorOpenClose = "NO"

     'select Max date

     Dim Proxy1 As ComboLoadClient = New ComboLoadClient()

     AddHandler Proxy1.SelectmaxdateCompleted, AddressOf LastPositionRunning

     Proxy1.SelectmaxdateAsync(" WHERE       ([Group].GroupName = '" & TxtUsername.Text & "' )")

 End Sub


  

[Meenu]

I found  the problem.But could not solve the issue

When repeating the execution when I changing the selection  in the following combo.After each selection change again timer control is
calling the handler _timerRealtime_Completed

Help pls

 

Private Sub Cmbn_SelectionChanged(sender As Object, e As System.Windows.Controls.SelectionChangedEventArgs) Handles Cmbn.SelectionChanged

        'realtime code added
        If ChkRealTime.IsChecked = True Then
            ChkRealTime.IsChecked = False
            ChkRealTime.IsChecked = True
            Exit Sub
        End If