roundup-mailgw appears to get confused when more than
one user have the same email address assigned (either
in address or in alternate_addresses).

When trying to attribute incoming mail to a user, the
gateway should try to disambiguate by means of mail
header uid. If it can't, the mail should bounce.

(We accidentally ran into this in a test tracker, where
multiple users used the same email address, but had
differing permissions set, causing some internal 
errors...)