Private Sub Worksheet_Change(ByVal rg As Range)
  Dim Nv, Ov, Lv
  If Intersect(rg, Range("C15:C18")) Is Nothing Or rg.Count > 1 Then Exit Sub
  If Len(rg) = 0 Then Exit Sub
  Application.EnableEvents = False:  Nv = rg:  Application.Undo:  Ov = rg
  If Ov = Nv Then rg = "": Application.EnableEvents = True: Exit Sub
  If Len(Ov) = 0 Then
    rg = Nv
  Else
    Lv = NvInOv(CStr(Nv), Ov): If Lv = False Then rg = rg & ";" & Nv Else rg = Lv
  End If
  Application.EnableEvents = True
End Sub


Function NvInOv(Nv$, Ov)
  Dim v, i&
  v = Split(Ov, ";"): NvInOv = False
  For i = LBound(v) To UBound(v)
    If v(i) = Nv Then
      v(i) = Empty: NvInOv = Replace(Join(v, ";"), ";;", ";"): v = Split(NvInOv, ";")
      If v(UBound(v)) = "" Then NvInOv = Left(NvInOv, Len(NvInOv) - 1)
      If Left(NvInOv, 1) = ";" Then NvInOv = Right(NvInOv, Len(NvInOv) - 1)
      Exit Function
    End If
  Next
End Function

Private Sub Worksheet_Change(ByVal Target As Range)
Dim newVal As String, oldval As String
    If Target.Cells.Count > 1 Then Exit Sub

    If Not Intersect(Target, Range("C15:C18")) Is Nothing Then
        Application.EnableEvents = False
        
        With Target
            newVal = .Value: Application.Undo: oldval = .Value
        
            If oldval Like "*" & newVal & "*" Then
                oldval = Replace(Replace(oldval, newVal, ""), ";;", ";")
                If Right$(oldval, 1) = ";" Then oldval = Left$(oldval, Len(oldval) - 1)
                .Value = oldval
            Else
                .Value = .Value & ";" & newVal
            End If
            
            If Left$(.Value, 1) = ";" Then .Value = Mid$(.Value, 2)
        End With
        
        Application.EnableEvents = True
    End If
End Sub