An apparatus and method for improving performance in high-availability systems are disclosed. In accordance with the illustrative embodiment, pages of memory of a primary system that are to be shadowed are initially copied to a backup system's memory, as well as to a cache in the primary system. A duplication manager process maintains the cache in an intelligent manner that significantly reduces the overhead required to keep the backup system in sync with the primary system, as well as the cache size needed to achieve a given level of performance. Advantageously, the duplication manager is executed on a different processor core than the application process executing transactions, further improving performance.