Microsoft say this about Winsock error 10056 - WSAEISCONN:

A connect request was made on an already-connected socket. Some implementations also return this error if sendto is called on a connected SOCK_DGRAM socket (for SOCK_STREAM sockets, the to parameter in sendto is ignored) although other implementations treat this as a legal occurrence.

Full details of Winsock TCP error 10056 - WSAEISCONN:

Berkeley description: A connect request was made on an already connected socket; or, a or sendmsg() request on a connected socket specified a destination when already connected.

WinSock description: Same as Berkeley, except WinSock doesn't support the sendmsg() function, and some WinSock implementations are not so strict as to require an application with a datagram socket to "disconnect"--by calling with a AF_INET NULL destination address: INADDR_ANY (0.0.0.0), and port 0--before redirecting datagrams with or. On a datastream socket, some applications use this error with a non-blocking socket calling connect() to detect when a connection attempt has completed, although this is not recommended since some WinSocks fail with WSAEINVAL on subsequent calls.