Sieve: Avoiding duplicates in a folder while keeping in Inbox

[Nick Urbanik via dovecot]

Dear Folks,

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"
"pay...@paypal.com.au" {
        if not duplicate {
                fileinto "paypal";
        }
        keep;
}

Please can anyone enlighten me?
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