A graphics processing system comprising: a plurality of processing units for processing tasks, each processing unit being configured to process a task independently from any other processing unit of the plurality of processing units; a check unit operable to form a signature which is characteristic of an output of a processing unit on processing a task; and a fault detection unit operable to compare signatures formed at the check unit; wherein the graphics processing system is configured to process each task of a first type first and second times at the plurality of processing units so as to, respectively, generate first and second processed outputs, wherein the check unit is configured to form first and second signatures which are characteristic of, respectively, the first and second processed outputs, and wherein the fault detection unit is configured to compare the first and second signatures and raise a fault signal if the first and second signatures do not match; and each task of a second type is processed only a first time at the plurality of processing units so as to generate a respective single processed output, and characterised in that: data is maintained identifying which processing units of the plurality of processing units each task has been allocated to, wherein said data is consulted on allocating a task of the first type to a processing unit for processing for a second time, and, in response, that task is allocated for processing for the second time to any processing unit of the plurality of processing units other than the processing unit to which that task was allocated for processing for a first time. a plurality of processing units for processing tasks, each processing unit being configured to process a task independently from any other processing unit of the plurality of processing units; a check unit operable to form a signature which is characteristic of an output of a processing unit on processing a task; and a fault detection unit operable to compare signatures formed at the check unit; wherein the graphics processing system is configured to process each task of a first type first and second times at the plurality of processing units so as to, respectively, generate first and second processed outputs, wherein the check unit is configured to form first and second signatures which are characteristic of, respectively, the first and second processed outputs, and wherein the fault detection unit is configured to compare the first and second signatures and raise a fault signal if the first and second signatures do not match; and each task of a second type is processed only a first time at the plurality of processing units so as to generate a respective single processed output, and characterised in that: data is maintained identifying which processing units of the plurality of processing units each task has been allocated to, wherein said data is consulted on allocating a task of the first type to a processing unit for processing for a second time, and, in response, that task is allocated for processing for the second time to any processing unit of the plurality of processing units other than the processing unit to which that task was allocated for processing for a first time.