Why are there two mature sequences of microRNAs in miRBase entries?
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most frequently predominates, but I have personally found microRNAs in which both components occur in similar amounts (unpublished).