Citing contemporaneous (interlaced?) preprints
Cite relevant literature
Since you have the option of adding a citation to a relevant source, you should do it. It improves the paper by making it a more useful reference, and comparing and contrasting the results might also be useful.
Priority
If relevant, use a phrase like &quot;A very recent preprint A claims this and that.&quot;, or even, if your results are very similar and you really need to emphasize priority, &quot;During peer review of this article a preprint A was published. The preprint...&quot;.