ListBox ObservableCollection duplicating - wpf

Question

I have a WPF application which has a listbox bound to an ObservableCollection which retrieves it's data from a Database. I am attempting to have the ListBox data refreshed every minute through the use of a DispatcherTimer.
Dim dispatcherTimer As DispatcherTimer = New System.Windows.Threading.DispatcherTimer
AddHandler dispatcherTimer.Tick, AddressOf getRoomMeetingDetails
dispatcherTimer.Interval = New TimeSpan(0, 2, 0)
dispatcherTimer.Start()
Which calls the getRoomMeetingDetails method as follows.
Public Sub getRoomMeetingDetails()
If Not My.Settings.rbConn = Nothing And _
Not gl_rmName = Nothing Then
Dim sqlConn As New SqlConnection(My.Settings.rbConn)
Dim sqlquery As String = "SELECT *" & _
"FROM table " & _
Dim sqlCmd As New SqlCommand(sqlquery, sqlConn)
sqlConn.Open()
Dim dr As SqlDataReader
dr = sqlCmd.ExecuteReader
While dr.Read
roomMeetingList.Add(New meetingDetails() With {.eMeetingId = dr.Item("dId")})
End While
End If
End Sub
I then have my two classes for the Collection as follows (I am very new to ObservableCollections and have tried my best to model my code off the MSDN examples, so if this isn't the best method to use to achieve what I am trying to achieve, or can be done easier, please let me know)
Public Class MeetingList
Inherits ObservableCollection(Of meetingDetails)
Private Shared list As New MeetingList
Public Shared Function getList() As MeetingList
Return list
End Function
Private Sub New()
AddItems()
End Sub
Public Shared Sub reset(ByVal rmName As String)
list.ClearItems()
list.AddItems()
End Sub
Private Sub AddItems()
End Sub
End Class
Public Class meetingDetails
Implements INotifyPropertyChanged
Public Sub New()
End Sub
Public Property eID() As String
Get
Return _eID
End Get
Set(ByVal value As String)
_eID = value
OnPropertyChanged("eID")
End Set
End Property
Private _eID As String
Public Event PropertyChanged As PropertyChangedEventHandler _
Implements INotifyPropertyChanged.PropertyChanged
End Class
What is happening is when the DispatcherTimer is called every minute, the ListBox data is duplicated which I believe is because the getRoomMeetingDetails method is adding all of the SQL results on every tick. How can I refresh the ListBox with only new data or data changes from the table?
I am really struggling to work out where I am going wrong and what needs to be added/removed for this to work.
If there is any details I am missing please let me know.
Matt

Answer 1

Either you clear all the data in the listbox before adding them again or you do a check on the collection. I assume your eID is the primary key? the do something like this:
if ( roomMeetingList.Where ( entry => entry.eID == dbID ).Count () == 0 ) {
// add
}
C# code, but it shows the idea
developerFusion's convert made this VB:
If roomMeetingList.Where(Function(entry) entry.eID = dbID).Count() = 0 Then
' Add
End If