A 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).  In this paper we address the
problem of exactly-once delivery to mobile clients when messages are
replicated.  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.