Preventing UserForm From ReDisplaying

I have this code,


If Not Intersect(Target, Range("h15")) Is Nothing Then

If Target.Count > 1 Then Exit Sub

If Target.Value = "Ratio" Then
If UF.Visible = False Then
UF.Show
End If
Else
If Target.Value = "Acid Test" Then
Sheet2.Visible = xlSheetVisible
Sheet1.Visible = xlSheetHidden
Sheet2.Activate
Application.Goto Sheet2.Range("A1"), _
Scroll:=True
End If
End If
End If

The code works fine. However, if the target cell contains the word "Ratio", and the user opens the drop down menu in then cell and selects the work "Ratio" again, the userform displays. I only would like the userform to display if the cell went from being blank to "ratio" or from being another value to "ratio".