IRSEND and IR emitter flashes

Normally for each key sent by IRSEND, I am able to see the IR emitter visibly flash twice. Sometimes it will flash only once, and occasionally it will not flash at all (failing). I presume that each flash is a transmit of the key with the sequence number incremented for the 2nd send. I do not yet know why or exactly where this is failing. I would like in the meantime to increase the number of these sends but have not yet found where this could be done.

Can someone please explain where and how this flow works and can be configured? I would particularly like to know if this is a LIRC feature or a driver feature.

Thank you for your help.
Bryan Hunter