I have a code that i'm using for one column but i want to use the same code
on multiple columns, how do i change the code so it repeats for three more
columns, currently just 21, would like it to be for 22, 23 & 24 too. I tried
pasting it 4 times but i kept on getting different error messages. Can
anyone help??

Code I'm using:

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
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 = 21 Then
If oldVal = "" Then
'do nothing
Else
If newVal = "" Then
'do nothing
Else
Target.Value = oldVal _
& ", " & newVal
End If
End If
End If
End If

exitHandler:
Application.EnableEvents = True
End Sub