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In this exercise, the following hint is given -

'Use devoir, repeated action in the past'

so the imparfait is required. 

In this context, it would not mean 'was supposed to'.

But you would use the P. Composé in the following single event example-

Hier, j'ai dû me lever tôt car on partait en vacances = Yesterday I had to get up early as we were going on holiday.