You should not get 0 bytes as a return value for EndSend() unless you  
actually called BeginSend() with 0 bytes.  If you're using a  
connection-oriented socket (eg TCP), no data should be sent at all in this  
case, though you will get the successful completion with 0 bytes as the  
return value.  With a connectionless socket (eg UDP), a 0 length datagram  
will be sent.

You don't say what kind of socket you're using.  Usually one uses SendTo()  
instead of Send() with UDP, but only UDP supports the idea of "packets".  
So your post is self-contradictory, making it difficult to know exactly  
what it is you're doing.

If you are actually using UDP, then keep in mind that having the same  
datagram received twice is a known, correct possible occurrence.  With  
UDP, you can get a given datagram once, not at all, or multiple times.

If you cannot figure out how it is that you're calling BeginSend() with 0  
bytes, you might post the code here to see if someone can help you  
identify the problem.  Post only a concise-but-complete example of the  
code that reliably reproduces the problem.  Don't include a bunch of stuff  
that has nothing to do with the bug you're dealing with.

Pete