We propose to use a duplication based approach in scheduling tasks for a heterogeneous cluster of PCs. In duplication based scheduling, critical tasks are redundantly scheduled to more than one machine in order to reduce the number of inter-task communication operations. The start times of the succeeding tasks are also reduced. The task duplication process is guided given the system heterogeneity in that the critical tasks are scheduled or replicated in faster machines. The algorithm has been implemented in our prototype program parallelization tool for generating MPI code executable on a cluster of Pentium PCs. Our experiments using three applications have indicated that the heterogeneity of a PC cluster, being an inevitable feature, is indeed useful for optimizing the execution of parallel programs.