Sieve: Avoiding duplicates in a folder while keeping in Inbox

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

Please can anyone enlighten me?