Before revision:  To relax the constraints on the primal and dual method , a more general result is derived that enables the use of a recently developed mass conserving mixed stress discretisation to design equilibrated fluxes that yield pressure-independent guaranteed upper bounds for any pressure-robust ( but not necessarily divergence-free) primal discretisation.
After revision:  To relax the constraints on the primal and dual method , a more general result is derived that enables the use of a recently developed mass conserving mixed stress discretisation for the design of equilibrated fluxes and to obtain pressure-independent guaranteed upper bounds for any pressure-robust ( but not necessarily divergence-free) primal discretisation.