Any person interested may, at any time within two years, by petition or bill, contest the validity of the will probated without notice; and an issue shall be made up and tried as other issues to determine whether the writing produced be the will of the testator or not. If some person does not appear within two years to contest the will, the probate shall be final and forever binding, saving to infants and persons of unsound mind the period of two years to contest the will after the removal of their respective disabilities. In case of concealed fraud, the limitation shall commence to run at, and not before, the time when such fraud shall be, or with reasonable diligence might have been, first known or discovered.