A  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  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.
I usually follow the rule that I only omit a duplicate citation if there has not been an intervening citation since the last time I cited it. Moreover, if it’s been a “long time” since the previous citation, I will repeat it, even if no other citation has occurred in between.
I try and teach students to use a " ( ) -> " rule: Make it clear where some text or idea from someone else starts, where it ends, and where you found it. If you are quoting, the parentheses are a quotation mark and the arrow is the reference:
"Blah fasel" (X, 2017, p. 13)
If you are paraphrasing, it works like this, using the references as combined begin or end markers and a reference. :
X has stated (2017, p. 13) that Humpty Dumpty did this and that. X emphasized (p. 42) that the falling down was part of a conspiracy. But X does make clear that the reason way because he was using a mobile phone (p. 117). It was later determined by .....
This way you are making it very easy for your READERS to find the point at which they can verify the things that you are stating. Write for your readers, not to save yourself some keystrokes.