[BUG]: Duplicate and missed departures after manually removing convoy

[freddyhayward]

Steps to reproduce:
1. Have at least 2 convoys waiting at a stop
2. Remove any convoy scheduled to depart before the latest one, either by selling it, withdrawing it, altering its schedule position, or reassigning it to a different line.
3. Wait for another convoy to arrive at the stop before the now-removed convoy was scheduled to depart
4. Observe that neither the new convoy nor any other convoy have filled the now-empty departure slot occupied by the now-removed convoy
5. Observe that the new convoy now occupies the same slot as another convoy, so that they are scheduled to depart at the same time
6. Observe that the incorrect pattern repeats for a number of departures before correcting itself.