Error while file download using winsock

We are using vb6 winsock for downloading files.It works fine for lot of our customers. But only for some 5 customers the file does not get downloaded properly. The request is like shown below

GET http://www.abc.com/files/Web_236.txt HTTP/1.0
Host: www.abc.com
Accept: *.*, */*
Range: bytes=0-641
User-Agent: Winman Software (P) Ltd.
Referer: www.abc.com


GET http://www.abc.com/files/Web_236.txt HTTP/1.0
Host: www.abc.com
Accept: *.*, */*
Range: bytes=642-1283
User-Agent: Winman Software (P) Ltd.
Referer: www.abc.com

The problem is that in some system where Full file part is downloaded 2 times. Why does this error happens?