Private Sub Worksheet_Change(ByVal Target As Range)
' Developed by Contextures Inc.
' www.contextures.com
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
If Target.Count > 1 Then GoTo exitHandler

On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler

If rngDV Is Nothing Then GoTo exitHandler

If Intersect(Target, rngDV) Is Nothing Then
   'do nothing
Else
  Application.EnableEvents = False
  newVal = Target.Value
  Application.Undo
  oldVal = Target.Value
  Target.Value = newVal
  If Target.Column = 2 And Target.Row = 10 Then '此处选定第2列第10行(B10)的单元格为需要设置下拉多选的单元格
    If oldVal = "" Then
      'do nothing
      Else
      If newVal = "" Then
      'do nothing
      Else
        If InStr(1, oldVal, newVal) <> 0 Then  '重复选择视同删除
          If InStr(1, oldVal, newVal) + Len(newVal) - 1 = Len(oldVal) Then '最后一个选项重复
            Target.Value = Left(oldVal, Len(oldVal) - Len(newVal) - 1)
          Else
            Target.Value = Replace(oldVal, newVal & ",", "") '不是最后一个选项重复的时候处理逗号
          End If
        Else '不是重复选项就视同增加选项
        Target.Value = oldVal & "," & newVal
'      NOTE: you can use a line break,
'      instead of a comma
'      Target.Value = oldVal _
'        & Chr(10) & newVal
        End If
      End If
    End If
  End If
End If

exitHandler:
  Application.EnableEvents = True
End Sub