filtering outgoing duplicates?

Anyone have a tip/suggestion on how to add a check for duplicates, before smartlist sends out a message? Sometimes list users mention the list address twice, in To: and Cc: lines for example, and it'd be nice to enusre that only one copy of the message goes out. I know that it is easy to set up a message-id cache in procmail - just wondering if there's a straightforward method for adding this filter to smartlist?