DAQmx RegisterEvent error

Hi, I have following code:

void SetSingle (TaskHandle AITask)
{
DAQmxCfgSampClkTiming (AITask, "OnboardClock", numberOfSamp*2, DAQmx_Val_Rising, DAQmx_Val_FiniteSamps, numberOfSamp);
DAQmxRegisterDoneEvent (AITask, 0, fcn_measure_finite, NULL); 
}

for the first time its called its fine, but when I call this again, it gives me this error:

Done Event Registration has failed because event is already register within the task. Unregister the event before registering it again. I searched internet a lot but havent find anything...any ideas how to unregister event? 

Thanks.