Why are there miRNA genes on both strands at the same location?

Answer: 

Most of the VectorBase miRNA genes are predicted computationally, by aligning Rfam covariance models against the genome. Some miRNA genes, as a consequence of the miRNA's secondary structure, have symmetrical properties that generate credible alignments on both strands. In the absence of further evidence, we display both as separate genes; it is most likely that only one is correct, but we have no way of discerning which.

The description of such miRNA genes contains the following text: "NOTE: The automated annotation of this gene could not determine the strand, so a version exists on both strands."