In this paper, we propose an O(v²) scalable duplication based algorithm (RT-SDA) for scheduling precedence constrained real-time tasks on heterogeneous multiprocessors. This models a network of workstations, with processors of varying computing power. The algorithm takes the heterogeneities of both computation and communication of the multiprocessor system into account. RT-SDA employs selective task duplication to reduce the start time of the real-time tasks in the job, thereby increasing the guarantee ratio of the real-time application. Moreover, our scheme is scalable in that the application can be scheduled even if the available number of processors is less than the required number of processors. Compared to the existing scheduling algorithms in the literature, RT-SDA offers better schedulability in terms of a higher guarantee ratio.