Following advice of MSDN user, I have implemented this solution:
I moved the script log task in the SSIS's event onPostExecute, I switched on tasks's property DisableEventHandlers.
If the single task don't have its onPostExecute event, the execution don't have problems and parent's event is fired regularly (1 time); Otherwise the event is passed to parent container and the event is fired 2 times.
So the sequence container also have DisableEventHandlers event disabled, but the parent event onPostExecute, is also fired
Someone known a work around for this behavior?