Lotus script if condition not do exit sub

I try to show messagebox only 1 time if user input "number < somevalue(days)" for warning to user input a new value that's bigger than somevalue. I try to check my code but I didn't find a problem why it's do repeat to show messagebox more than 1 times. [this code I code on Existing event]

Sub Exiting(Source As Field)
    Dim workspace As New NotesUIWorkspace
    Dim session As New NotesSession
    Dim uidoc As NotesUIDocument
    Dim db As NotesDatabase
    Set uidoc = workspace.CurrentDocument
    Set doc = uidoc.Document

    Dim diff As Long

    Set ComDate = doc.getFirstItem("CompletionDate").dateTimeValue
    Set DueDate = doc.GetFirstItem("DueDate").dateTimeValue

    diff = ComDate.TimeDifference(DueDate)  
    Dim days As Integer
    days=diff/86400
    Dim number As Integer
    Dim num As String
    num=uidoc.FieldGetText("DeliveryTiming")
    number= Cint(num)
    If(number<days) Then 
        Messagebox( "Please specify your request DeliveryTiming?" ),16,"Required input"
        Call uidoc.GotoField( "DeliveryTiming" )
        Exit Sub
    End If
End Sub