dovecot: enable the duplicate Sieve extension

Useful for preventing message duplication when the recipient is also part of an alias included in Cc (e.g. foutrelis@ + infrastructure@).

added 3 commits

• 7fc66e61...35927e41 - 2 commits from branch master
• c379884b - dovecot: enable the duplicate Sieve extension

Compare with previous version

mentioned in commit 395af17e

merged