Notes/Domino 4 and 5 Forum

I have a field "etype" on my form which is mandatory. Can anyone help me how to perform this mandatory check at field level. I tried following code at Exiting event of my field "etype". It works fine only if exit the field with mouse click. However, if i use the TAB key to navigate to next field the message appears three time before giving the focus to the mandatory "etype" field.
Help please!!!
Sub Exiting(Source As Field)
Dim workspace As New NotesUIWorkspace
Dim uidoc As NotesUIDocument
Dim curdoc As NotesDocument
Set uidoc = workspace.CurrentDocument
Set curdoc = uidoc.Document
Call uidoc.Refresh

If curdoc.etype(0) = "" Then
Messagebox "Select Type of item"
Call uidoc.GotoField("etype")
Exit Sub
End If
End Sub