You don't _receive_ duplicate messages when posting from Gmail; one is the
message you sent, and in most mail reders would be in the "sent" folder.
Gmail puts these in with the incoming emails. The duplicate is the one
received via the reflector (if you select that option).
If you read your mail offline with an email client on your PC, none of this
applies and Gmail behaves the same as any other provider.
Perhaps there is a case for Groups.io to issue a new message ID for every
post and forget about the option.