1. A data processing apparatus comprising: processing circuitry configured to perform a data processing operation in response to a micro-operation, the processing circuitry having an output register configured to store a result of the data processing operation; and control circuitry configured to detect whether a current micro-operation to be processed by the processing circuitry would give the same result as the last valid micro-operation to be processed by the processing circuitry before the current micro-operation; wherein if the control circuitry detects that said current micro-operation would give the same result as said last valid micro-operation, then the control circuitry is configured to prevent the processing circuitry processing the current micro-operation so that the output register is not updated in response to the current micro-operation, and to control the processing circuitry to output the value stored in the output register as the result of the current micro-operation.