is validation and train set should be all different files? Let say I have train set and validation set
if 'A' included in the train set. 'A' should not be include in the validation set? or some is ok?
Validation set is basically to understand how your model is behaving in terms of over-fitting, and under-fitting, and also to find the best set of hyper-parameters for your algorithm. If you use some parts of the same training data on your validation set, then this hypothesis would not be justified. 
Hence, it is suggested to split your data set into train/validation/test set without any overlapping of the data.