Do you have to double board your ceiling when you replace it in your ground floor flat as it has single 10mm plasterboard on it but I've been told if I take it down it has to be double boarded?

There may be regulations applying to your building that mean, when work is done to parts of it, those parts must be brought up to a higher standard. This requirement could be because of local authority regulations or even your lease agreement, depending on your situation. Ask what is the reason for the requirement.
Double boarding is usually done for acoustic reasons.