[exim] Exim sends message twice if alias points to itself

Greetings,

I have switched from Sendmail to Exim and would like to handle this situation:

In /etc/aliases:

userlist: :include:/path/to/a/file

Whereas the /path/to/a/file contains this:

userlist
user1
user2

I.e., userlist is both local account and forwarder. In such a case,
any email sent to

userlist@???

comes *twice* to all the mailboxes mentioned in /path/to/a/file.

Sendmail handles that, suppressing duplicate messages. How can one
suppress the duplicates in Exim without creating an alternate local
accounts with names different from forwarders?

Thanks.