this is from the ActionScript documentation for NetGroup.post():
All messages must be unique. A message that is identical to one posted earlier might not be propagated. Use a sequence number to make messages unique.
the *reason* why messages must be unique is: peers gossip with each other about the postings each has. each posting is identified by its messageID, which is the SHA256 of the AMF serialization of the posting. peers attempt to retrieve new postings they don't have already from peers that announce them. a second posting of a message will have the same messageID, so peers will believe they already have the message and not retrieve/propagate it again.
note that the documentation says duplicates *might* not be propagated -- messages eventually time out, but the timeout is not specified and may change in the future.
well-behaved senders SHOULD assume that duplicates will not be propagated and ensure all messages are unique. well-behaved receivers SHOULD assume that duplicates may be received (eventually) and, if that's a problem, take extra precautions to filter duplicates.
Ok thank you very much. This should solve my problem