[Exim] trashing self messages

I'm trying to set up a mini-mailinglist just with aliases.

like: aliases-file:
alluser: user1, user2, user3

Now "user1" sends a mail to "alluser" but since
he's also in the alias he receives his own message.

Can someone please give me a hint how to suppress this
message? My first though was something like to trash all
messages where sender and receipent are the same.

Any thoughs?
--
Torsten