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Dependable real-time systems typically consist of tasks of mixed-criticality levels with associated fault tolerance (FT) requirements and scheduling them in a fault-tolerant manner to efficiently satisfy these requirements is a challenging problem. From the designers' perspective, the most natural way to specify the task criticalities is by expressing the reliability requirements at task level, without having to deal with low level decisions, such as deciding on which FT method to use, where in the system to implement the FT and the amount of resources to be dedicated to the FT mechanism. Hence, it is extremely important to devise methods for translating the high-level requirement specifications for each task into the low-level scheduling decisions needed for the FT mechanism to function efficiently and correctly. In this paper, we focus achieving FT by redundancy in the temporal domain, as it is the commonly preferred method in embedded applications to recover from transient and intermittent errors, mainly due to its relatively low cost and ease of implementation. We propose a method which allows the system designer to specify task-level reliability requirements and provides a priori probabilistic scheduling guarantees for real-time tasks with mixed-criticality levels in the context of preemptive fixed-priority scheduling. We illustrate the method on a running example.