By default special services can be "overloaded" with volunteers scheduled twice in quick succession, since often this is needed to complete the schedule during busy times like Holy Week. You can override this behavior by clicking on a service in the schedule skeleton window (before clicking the "Commence Scheduling..." button), and then clicking the "Edit Service..." button. Then uncheck the "Overload volunteers' schedules for this service if necessary" button, which is on for all special services by default.
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