When is it convenient dropping duplicates when performing Time Series Prediction?
According to me, time-series ML Model are bit different than other routine ML  models. As time-series model is based upon the sequence of previous values, dropping a Date may impact your solution.

Specially in this case where sale is definitely dependent upon the particular day.
For eg consider store 38 is located in Area where on "2017-03-15" is holiday. So sale of store 38 will be comparatively high in comparison to other store and so on.

Rest you can try two different models one with Date and another without Date and compare the results as well.