I am in the process of learning Sieve, and want to be able to keep an
email in Inbox, while also keeping one copy in another folder.
However, the same mail is copied from Inbox multiple times when I run
sieve-filter -Wev .dovecot.sieve Inbox

My attempt to prevent this fails:

...
} elsif header :comparator "i;octet" :contains "From" "[email protected]" {
        if not duplicate {
                fileinto "paypal";
        }
        keep;
}

Another way of putting this is: how to tell Sieve to refuse to put a second copy of an email into a folder if it is already there?

Please can anyone enlighten me?