In the miRBase database entries generally show a stem loop sequence and two mature sequences. For example the entry for hsa-mir-15a gives three sequences: (1) the Stem-loop sequence hsa-mir-15a, (2) Mature sequence hsa-miR-15a-5p, and (3) Mature sequence hsa-miR-15a-3p. What is the difference between the two mature sequences? Does the stem loop structure divide into two different miRNA species?
I am not sure whether it is made clear in the Wikipedia article on microRNAs, but, yes, there are two possible mature mir species that can be generated from the stem-loop precursor — one from each side of the stem — although most often one greatly predominates.
The diagram in Wikipedia shows the generation of a single miRNA:
However, the entry mentioned in the question (hsa-mir-15a) shows that although the 5′-stem component (UAGCA…) is most abundant in this case, there is a small amount of the 3′-stem component (CAGGC…).
In Drosophila melanogaster a single stem component (more often from the 3′-stem, but also from the 3′-stem) most frequently predominates, but I have personally found microRNAs in which both components occur in similar amounts (unpublished).