Support scheduling conflict alerts

I would like to be alerted when scheduling someone who has already been scheduled that day. We have many people scheduling, and they often do so blind. It is a pain to sort through all the schedules to avoid all the conflicts.

I don't know how easy this would be to implement, but right now it is the prime feature I care about, and it would be great to see it here, so I didn't have to pay for the proprietaries (planningcenteronline, worshipplanning, etc.).