Workflows are the de facto standard for managing business processes and allow businesses to automate interactions between business locations and partners residing anywhere on the planet. This, however, requires the workflows to be executed in a dynamic environment, where device and communication failures occur frequently, making availability a key concern. In this work, we propose the replicated execution of workflows for ensuring availability in the presence of failures. The replicated execution has to yield the same result as a non-replicated execution of that workflow. Thus, we formally define Single-Execution-Equivalence and present a replication scheme that adheres to this definition. We implement a proof-of-concept using an open-source workflow engine for demonstrating the compatibility with current workflow technology. Our evaluations on Amazon EC2, OpenStack, and PlanetLab show that workflow replication ensures availability while being scalable and incurring low overhead in terms of execution time.