Like this:
Private Sub Worksheet_Change(ByVal Target As Range) 'Code by Sumit Bansal from https://trumpexcel.com ' To allow multiple selections in a Drop Down List in Excel (without repetition) Dim OldValue As String Dim NewValue As String If Target.CountLarge > 1 Then Exit Sub If Intersect(Range("C:C,L:M"), Target) Is Nothing Then Exit Sub On Error Resume Next If Target.SpecialCells(xlCellTypeAllValidation) Is Nothing Then Exit Sub If Target.Value = "" Then Exit Sub Application.EnableEvents = False On Error GoTo ExitSub NewValue = Target.Value Application.Undo OldValue = Target.Value If OldValue = "" Then Target.Value = NewValue ElseIf InStr(1, OldValue, NewValue) = 0 Then Target.Value = OldValue & ", " & NewValue Else Target.Value = OldValue End If ExitSub: Application.EnableEvents = True End Sub