datatable select not found [2005]

[luckyboy]

hi all.i would like to know, why datatable.select(condition), sometimes it found records, but sometimes not found a record.
here this is an example of my datatable:

ID itemID
1 2
1 3
1 1
1 4
1 2 I don't want this record
2 3
2 4
2 1
2 3 I don't want this record too.

I would like to insert without a duplicate value.

and here is my datatable.select function(purpose of the function is to search)

    Private Function SearchDataTable(ByVal dt As DataTable, ByVal condition As String) As Integer
        Dim ReturnValue As Integer
        ReturnValue = dt.Select(condition).Length
        Return ReturnValue
    End Function

    Private sub cmd_Click()
           if searchdatatable(dt, "id=" & id & " and ItemID=" & txtItemID.text)=0 then
             ........
             ........
             ........
           End If
          ........
          ........
          ........
    End Sub

anyidea please, many thank for your help.