[Exim] Duplicate deliveries on forward in exim4

Hi all,

I got problems with some users that have multiple mailboxes
and recursive forwards. For example:

* user michael foward to: michael, andrea
* user andrea forward to: andrea, michael

When a message arrive to michael, it arrive once to michael,
ok but it arrives two times to andrea.

Is there anyway to restrict or remove duplicate envelope-to
generated by forward router? I tried check_ancestor, but
without success.

This is my router conf:

virtual_forward:
driver = redirect
allow_defer
allow_fail
check_ancestor
data = ${lookup mysql{MYSQL_FORWARD}}
user = mail
group = mail
file_transport = address_file
pipe_transport = address_pipe
reply_transport = address_reply


Please, reply with cc: fabio@???

Thanks,
Fabio