I have a Userform which updates a worksheet with the following code. I would
like it to check to see if the txtOrderNumber has already been entered in the
MasterList before copying the data to the MasterList with a message that lets
the user know that the Order Number has already been used.

Private Sub cmdAdd_Click()
Dim iRow As Long
Dim ws As Worksheet
Set ws = Worksheets("MasterList")

'Find first empty row in MasterList
iRow = ws.Cells(Rows.Count, 1) _
.End(xlUp).Offset(1, 0).Row

'Check for a Order number
If Trim(Me.txtOrderNumber.Value) = "" Then
Me.txtOrderNumber.SetFocus
MsgBox "Please enter the Order Number"
Exit Sub
End If

'copy the data to MasterList
ws.Cells(iRow, 1).Value = Me.txtOrderStatus.Value
ws.Cells(iRow, 2).Value = Me.txtOrderNumber.Value
ws.Cells(iRow, 5).Value = Me.txtOrderDate.Value

'Clear the form
Me.txtOrderStatus.Value = ""
Me.txtOrderNumber.Value = ""
Me.txtOrderDate.Value = ""

Me.txtOrderStatus.SetFocus

End Sub