If the law already requires you to do something i.e. its an existing legal obligation e.g. giving evidence in court due to a subpoena (i.e. a court order requiring you to give evidence) then you cant use this as fresh consideration in support of a new contract.
For example if the court requires you to give evidence in a court case and then a lawyer says that they will pay for your travel to the court case so long as you give evidence then this is not valid and you would not be entitled to the travel as going to court is an existing obligation and therefore you have not done anything different to what is legally required and thus it is “insufficient”.