Without support for copy, the ingo user interface and the sieve rules 
can say different things, and some sequences of rules are impossible 
to code with ingo.  Redirect and fileinto are affected. Example:

I tell ingo to redirect to foo@example.com and I do not check Stop checking.

Ingo writes:  redirect "foo@example.com"; keep;

The explicit keep sends a copy to inbox and further rules are ignored!

Ingo should write:  redirect  :copy "foo@example.com";

That does an implicit keep and further rules are checked as ingo implies.

http://www.faqs.org/rfcs/rfc3894.html



This is similar to procmail's 'c' flag (carbon copy).