Abstract: A wide-area distributed message delivery system can use replication to improve performance and availability. However, without safeguards, replicated messages may be delivered to a mobile device more than once, making the device's user repeat actions (e.g., making unnecessary phone calls, firing weapons repeatedly). Alternatively, they may not be delivered at all, making the user miss important messages. In this paper we address the problem of exactly-once delivery to mobile clients when messages are replicated globally. We define exactly-once semantics and propose algorithms to guarantee it. We also propose and define a relaxed version of exactly-once semantics which is appropriate for limited capability mobile devices. We study the relative performance of our algorithms compared to weaker at-least-once semantics, and find that the performance overhead of exactly-once can be minimized in most cases by careful design of the system.