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A particular scheduling problem over a single machine is considered. Jobs belong to different classes, and two jobs belonging to the same class are considered as completely equivalent. A sequence of due dates is specified for each class of jobs, and the serviced jobs, for each class, are assigned to the due dates according to the EDD rule. The service time of any job of a given class has to be selected within an interval of possible values (the highest possible one being the nominal service time). A deviation of the actual service time from the nominal value determines a cost that has a linear dependence on such a deviation. The cost function to be minimized is the sum of the total (weighted) tardiness cost and the overall service time deviation cost. The decision variables are those concerning job sequencing (taking into account the equivalence of jobs of the same class) and service times. In this paper, a solution to this problem is provided in terms of closed-loop strategies.