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...)

User email addresses should be unique.

I'm not sure what "When trying to attribute incoming mail to a user, the gateway should try to disambiguate by means of mail header uid." means, or how it's attempting to solve the above problem. *The* mail header that identifies the author of an incoming message is the "From" address (or, rarely, "Resent-From").

I've added a check to the userauditor to ensure than email addresses are unique.