You know this parameter:
Range: bytes=a-bwhere a - start byte, b - end byte (may be empty).
So, now about logs, look at this:
12.06.2016 15:14:28 92.*.*.141:51746 Requested GET /file1.mkv
12.06.2016 15:14:28 92.*.*.141:51749 Requested GET /file1.mkv
12.06.2016 15:14:28 92.*.*.141:51748 Requested GET /file1.mkv
12.06.2016 15:14:28 92.*.*.141:51748 Fully downloaded - 1.1 K @ 93.3 KB/s - /file1.mkv
12.06.2016 15:14:28 92.*.*.141:51748 Requested GET /file1.mkv
12.06.2016 15:14:33 92.*.*.141:51750 Requested GET /file1.mkv
12.06.2016 15:14:51 92.*.*.141:51752 Requested GET /file1.mkv
12.06.2016 15:15:06 92.*.*.141:51753 Requested GET /file2.mkv
12.06.2016 15:15:06 92.*.*.141:51755 Requested GET /file2.mkv
12.06.2016 15:15:07 92.*.*.141:51756 Requested GET /file2.mkv
12.06.2016 15:15:07 92.*.*.141:51756 Fully downloaded - 1.1 K @ 111.9 KB/s - /file2.mkv
12.06.2016 15:15:07 92.*.*.141:51756 Requested GET /file2.mkv
12.06.2016 15:15:11 92.*.*.141:51754 Requested GET /file2.mkv
12.06.2016 15:15:20 92.*.*.141:51758 Requested GET /file3.mkv
12.06.2016 15:15:20 92.*.*.141:51759 Requested GET /file3.mkv
12.06.2016 15:15:21 92.*.*.141:51760 Requested GET /file3.mkv
12.06.2016 15:15:21 92.*.*.141:51760 Fully downloaded - 1.1 K @ 93.3 KB/s - /file3.mkv
12.06.2016 15:15:22 92.*.*.141:51760 Requested GET /file3.mkv
12.06.2016 15:15:27 92.*.*.141:51761 Requested GET /file3.mkv
12.06.2016 15:15:34 92.*.*.141:51762 Requested GET /file4.mkv
12.06.2016 15:15:35 92.*.*.141:51763 Requested GET /file4.mkv
12.06.2016 15:15:35 92.*.*.141:51764 Requested GET /file4.mkv
12.06.2016 15:15:35 92.*.*.141:51764 Fully downloaded - 1.1 K @ 79.9 KB/s - /file4.mkv
12.06.2016 15:15:35 92.*.*.141:51764 Requested GET /file4.mkv
12.06.2016 15:15:40 92.*.*.141:51765 Requested GET /file4.mkv
12.06.2016 15:15:49 92.*.*.141:51766 Requested GET /file5.mkv
12.06.2016 15:15:50 92.*.*.141:51767 Requested GET /file5.mkv
12.06.2016 15:15:52 92.*.*.141:51769 Requested GET /file5.mkv
12.06.2016 15:15:52 92.*.*.141:51769 Fully downloaded - 1.1 K @ 124.3 KB/s - /file5.mkv
12.06.2016 15:15:52 92.*.*.141:51769 Requested GET /file5.mkv
12.06.2016 15:15:59 92.*.*.141:51770 Requested GET /file5.mkv
12.06.2016 15:22:34 92.*.*.141:51752 Fully downloaded - 547.0 M @ 1.2 MB/s - /file1.mkv
12.06.2016 15:23:42 92.*.*.141:51754 Fully downloaded - 546.5 M @ 1.1 MB/s - /file2.mkv
12.06.2016 15:23:57 92.*.*.141:51761 Fully downloaded - 547.5 M @ 1.1 MB/s - /file3.mkv
12.06.2016 15:24:19 92.*.*.141:51770 Fully downloaded - 546.4 M @ 1.1 MB/s - /file5.mkv
12.06.2016 15:24:29 92.*.*.141:51765 Fully downloaded - 546.7 M @ 1.0 MB/s - /file4.mkv
This is very unconvinient to read and understand what happen, why files download twice, why this order of get. To understand this will be very helpfull add in log line this Range parameter, if it not empty. So it will help to see and understand what heppen here, because right now this situation in log looks useless.