show message box when a duplicate entry is entered into a userform textbox

Hello,

I have a userform that enters data in column A, I would like to get a msgbox response if the same entry has already been entered into column A, if this happens then the sheet resets itself and the user has to start again.

Private Sub CommandButton12_Click()


    ActiveWorkbook.Sheets("data").Activate

    Range("A1").Select
    Do
    If IsEmpty(ActiveCell) = False Then
        ActiveCell.Offset(1, 0).Select
    End If
    Loop Until IsEmpty(ActiveCell) = True
    ActiveCell.Value = TextBox15.Text
    
    ActiveCell.Offset(0, 1) = TextBox16.Text
    


End Sub

Hopefully its a very simple solution but it has me stumped!

thanks reg