What does dual metric and single metric mean？

[SKJKC]

    ret = OrderedDict()
    mAP_dual_metric = {iou: np.mean(x) for iou, x in aps_dual_metric.items()}
    mAP_single_metric = {iou: np.mean(x) for iou, x in aps_single_metric.items()}
    ret["bbox"] = {"AP@50IoU_dual_metric": mAP_dual_metric[50], "AP@50IoU_single_metric": mAP_single_metric[50]}
    return ret