how can I get my message box to display only one time after it has already been display?

[VBA]If LenB(Range("B11").Value) > 0 Then
If Year(frmPDCalc.Date1) <> Year(Range("B11").Value) Then
frmPDCalc.Ent1.Enabled = False
MsgBox "You may not register a FY" & Year(frmPDCalc.Date1) & " expense in a FY" & Year(Range("B11")) & " Expense Report Log.", vbExclamation, "PerDiem Traveler"
End If
End If
[/VBA]