Should I cite the reference if I have already cited it in the previous paragraph? For example, 


  A [1] is a dataset for training the model. 


I have cited once in this sentence.

Then in the second sentence somewhere in my paper,


  I would like to use A [1] as my experimental dataset.


Should I cite it again?
There may be field-to-field differences, but generally I think it depends on how far away "somewhere in my paper" is. If it is a just a few sentences away, in the same paragraph, then I don't think you need to cite again. In this context, it is probably clear that you are still talking about the first dataset.

However, if you are referencing it later in the paper, even in the next paragraph possibly, then I think you should err on the side of caution consider citing it again. I don't think that over-citation is as large a problem as some would say.

Lastly, there may be the case where it's clear that you will be using the same dataset throughout a paper. In that case, you could just make it clear that you anywhere you refer to "the dataset," it is clear what you are referring to. For example, "Throughout this paper we use the MNIST Handwritten Digit database [ref], and any reference to 'the dataset' or 'the data', will refer to this dataset." There is probably a less clunky way to phrase this.