Employed NMS twice

[siyilingting]

Hi, I find that NMS has been used twice.
First, to detect the object bbox, yolov3 employs the NMS on the all detected objects, and return bbox_xywh, cls_conf and cls_ids, as shown in YOLOv3/detector.py. After that in deepsort, another NMS is employed on the boxes as shown in deep_sort/deep_sort.py. What is the effect of the latter NMS?