Private Sub Worksheet_Change(ByVal Target As Range)
On Error Resume Next
With Target.Validation
If .formula1 = "" Then
.Add Type:=xlValidateList, formula1:=Target.Text
.InCellDropdown = True
.ShowInput = False
.ShowError = False
ElseIf InStr(.formula1, Target.Text) = 0 Then
.Modify , , , .formula1 & "," & Target.Text
End If
End With
End Sub