There is any way to prevent this? ( not using formail –D, please... )
Why not using formail -D? It's designed exactly for what you're asking.
That's like saying you want help connecting to the Internet, but you
won't accept any answers that involve using electricity.
There's no way to know reliably whether another copy of an incoming
message for Montoya is blind-carboned to Schumacher, nor vice versa,
ergo there's no way to know reliably while you're processing Montoya's
delivery whether or not Barrichello is already getting a copy. The only
answer is to do duplicate testing at Barrichello's end (and what if the
original is also blind-carboned to Barrichello and each of the users is
getting three copies?).
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