A multiprocessor computer system is provided having a multiplicity of sub-systems and a main memory coupled to a system controller. An interconnect module, interconnects the main memory and sub-systems in accordance with interconnect control signals received from the system controller. At least two of the sub-systems are data processors, each having a respective cache memory that stores multiple blocks of data and a set of master cache tags (Etags), including one cache tag for each data block stored by the cache memory. Each data processor includes a master interface for sending memory transaction requests to the system controller. The system controller processes each memory transaction and maintains a set of duplicate cache tags (Dtags) for each data processor. Finally, the system controller contains transaction execution circuitry for activating a transaction for servicing by the interconnect. The transaction execution circuitry pipelines memory access requests from the data processors, and includes invalidation circuitry for processing each writeback request from a given data processor prior to activation to determine if the Dtag index corresponding to the victimized cache line is invalid. Thereafter, the invalidation circuitry activates writeback requests only if the Dtag index is not invalid and cancels the writeback request if the Dtag index is invalid.