audit_user_fields() in the userauditor detector checks for duplicate alternate addresses 
using:

    check_alts = [x for x in db.user.filter(None, {'alternate_addresses': address}) if x != 
nodeid]

`filter()` on a String property compiles to `ILIKE '%value%'`, so any address that's a 
substring of an existing alternate_addresses entry is rejected as a duplicate. For example, 
setting `ken@example.com` fails if another user has `sp_ken@example.com` in their alternates.

This bug has seemingly existed for as long as the `check_alt` line was added, but I confirmed 
it on version 2.4.0. Attached is a patch mitigating this by checking for exact matches post-
filter, sort of like how it's done in the `mailgw.py` function `uidFromAddress()`.

Hello Mattias:

Thanks for the patch and including a test as well.

I am busy tomorrow and will plan on committing it on wednesday
with credit given to you.