I am using this vba code to prevent double entry:
Code:
Private Sub BusinessName_AfterUpdate()
  Dim NewBusinessName As String
  Dim stlinkcriteria As String

  NewBusinessName = Me.BusinessName.Value
  stlinkcriteria = "[BusinessName] = " & "'" & NewBusinessName & "'"
  If Me.BusinessName = DLookup("[BusinessName]", "Sheet1", stlinkcriteria)
...