Adding the same task twice leads to errors.

If you add the same task twice that will lead to the task chain being broken. It is possible to check against this by adding the following [here](https://github.com/arkhipenko/TaskScheduler/blob/master/src/TaskScheduler.h#L629)

```CPP
     if (aTask.iScheduler == this)
         return;
```