How To Prevent Duplicate Records?

i am using a textbox in the main form to transfer the Yr/Mth into the table. (refer to qp.zip)

now my problem is, how do i prevent duplicate records into the table?

e.g: currently whenever i scroll ard the months control in the form, i will get duplicate Mth/Yr into the table. how do i prevent that from happening?

i only want to have updated data inside the Mth/Yr table instead of duplicated Mth/Yr

Public Sub PutInMonthlyRecords()

Dim sql As String
Dim Db As DAO.Database
Dim rs As DAO.Recordset
Dim f As Form
Dim MthYr As String

Set f = Forms!frmQpi

MthYr = f("txtMthYr")

sql = "SELECT * FROM [tblQpiMonthly] WHERE (([tblQpiMonthly].[Input MthYr] = 'MthYr' ));"

Set Db = CurrentDb()
Set rs = Db.OpenRecordset(sql)


If rs.RecordCount = 0 Then
If (f!txtTotalPF) = 0 Then
Exit Sub
End If

rs.AddNew
rs![Input MthYr] = f("txtMthYr")
rs![Monthly TotalPF] = f("txtTotalPF")
rs![Monthly Rejection] = f("txtTotalAvoid")
rs![Monthly TotalAvoid] = f("txtRejection")

rs.Update

Else
If (f!txtTotalPF) = 0 Then
rs.Delete

Exit Sub
End If


rs.Delete
rs.AddNew
rs![Input MthYr] = f("txtMthYr")
rs![Monthly TotalPF] = f("txtTotalPF")
rs![Monthly Rejection] = f("txtTotalAvoid")
rs![Monthly TotalAvoid] = f("txtRejection")

rs.Update

End If


Db.Close

End Sub