Python multicast receiver - duplicate packets?

Question

Python

There is a script that opens a socket and reads a multicast from it (taken from here )

import socket
import struct

MCAST_GRP = '224.1.1.1'
MCAST_PORT = 1234

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind(('', MCAST_PORT))
mreq = struct.pack("4sl", socket.inet_aton(MCAST_GRP), socket.INADDR_ANY)

sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

while True:
  print sock.recv(10240)

everything is fine until I run the same script in parallel on another multicast group, but the ports are the same, for example
rtp://224.1.1.1:1234
rtp://224.1.1.2:
1234 second, and the second for the first.

I tried to do it as in mcast.py , the result is similar.

Why does this happen and how to cure it?

UPD. It was solved by substituting the addresses of the multicast group in sock.bind

-sock.bind(('', MCAST_PORT))
+sock.bind((MCAST_GRP, MCAST_PORT))