VB6 not inserting entries in SQL database table with a Primary Key

Hi ,

I have the following code in VB6 which "dismantles a text file and writes it into a SQL database table.It worked fine for 2 months and suddenly stopped.Here's the code(part of it)
Private Sub Collect()
    Dim f As Integer
    Dim strSourceFile As String
    Dim iFirstColon As Integer
    Dim strTextLine As String
    Dim strFieldName As String
    Dim strValue As String
    Dim pcs As Integer
    Dim cn
    Dim RS
    Dim ipcs
    Dim Throughput
    Set cn = CreateObject("ADODB.Connection")
    Set RS = New ADODB.Recordset
   
   
        cn.Open "driver=SQL Server;server=(local);uid=;pwd=;database=CheckWeighers;"
        RS.Open "statex1", cn, adOpenDynamic, adCmdTable, adLockPessimistic
        strSourceFile = "C:/statex.txt"
            f = FreeFile
            Open strSourceFile For Input As #f
                Do Until EOF(f)
                    Line Input #f, strTextLine
                    Select Case Trim$(strTextLine)
                        Case "BLOCKBEGIN"
                            RS.AddNew
                        Case "BLOCKEND"
                        'On Error Resume Next 'This line is to Avoid duplicates  as I set Primary Key the Duration
                            RS.Update
                        Case Else
                            iFirstColon = InStr(strTextLine, ":")
                            strFieldName = Left$(strTextLine, iFirstColon - 1)
                            strValue = Mid$(strTextLine, iFirstColon + 1)
                            RS.Fields(strFieldName).Value = strValue
                    End Select
                Loop
            Close #f
         
        RS.Close
    cn.Close
    loadflexgrid
End Sub

so the SQL TABLE has a primary key on the Duration column.For my understanding if the recorset encounters a value for the duration the same as an existing one in SQL then it should skip the that entry and continue inserting the next entries.Unfortunately this is not happening.I am just wondering why??
What actuallythe story is that the text file is being updated every 2 minutes with new rows (might be 1 row or 100 rows) but the rest of the rows is still on the text file.What I would luike to achieve is to insert in SQL only the new rows in the text file.I am doing something wrong somewhere however I cannot explain why it worked up untill now.Maybe because I exhausted all the possible distinct values for the duration coulmn?An whatever Is coming in th etext file has got in the duration a value which is already present in the database.If I remove the primary key constraint from the table then the code will insert all the entries from the text file which will create plenty of duplicates.
What is wronmg??