Option Explicit
Public oldVal As String

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
If Not Intersect(Target, Range("C2:C5")) Is Nothing And Target.Cells.Count = 1 Then
     oldVal = Target.Value
End If
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Dim newVal As String
Application.EnableEvents = False
If Not Intersect(Target, Range("C2:C5")) Is Nothing And Target.Cells.Count = 1 Then
     newVal = Target.Value
         If newVal <> "" Then
             If InStr(1, oldVal, newVal) = 0 Then
                 If oldVal = "" Then
                     Target = newVal
                 Else
                     Target = oldVal & "," & newVal
                 End If
                 oldVal = Target.Value
             Else
                 Target = oldVal
             End If
         End If
End If
Application.EnableEvents = True
End Sub