Worksheet Popup Message Shown 5 Times

I'm using the code below to form part of staff resource spreadsheet, but I'm having a little difficulty with a piece of the script.

Code:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rInt As Range, res As Variant
Dim cell As Range

Sheets("Input").Protect "password", UserInterFaceOnly:=True, AllowFiltering:=True

[Code] ....

The piece of the script which I'm having difficulty with is this:

Code:
For Each cell In Range("B7:B400")
If cell.Value = "No" Then
MsgBox "If " & cell.Offset(0, 1).Value & " has left R&D, please remember to delete any future resource forecasts"
End If
Next

When the cell value has changed to 'No' the pop up message appears, but for some strange reason the message is shown 5 times, despite their being only one record.The other problem I have is if I add another record and change the value to 'No' in column B, the message is shown twice, once for the previous record and the second for the new.