[Exim] Envelope-To with smartuser rewrites

I have this transport:

all_groupname:
  driver        = smartuser
  domains       = localhost
  prefix        = all-
  condition     = ${if ={${originator_uid}}{MY_USERID} {yes}{no}}
  rewrite       = true
  headers_add   = "X-List: all-${local_part}.globnix.org (private)"
  new_address   = ${perl{all_group}{$local_part}}

And the all_group() function will return the usercodes of all members of
the specified group. For the catch-all real account 'users' this works
well.

However, I've just done some tests with some minor system groups, where,
eg, one of the members is me, and one is an alias which rewrites to me.
So, a mail from me to <all-staff@localhost> expands to 'root' and 'phil'
and 'root' indirectly (via 'sysadm') maps to 'phil'.

The delivering director uses a transport with 'envelope_to_add' set, so
I get to see this:
-----------------------------< cut here >-------------------------------
Envelope-to: all-staff@localhost,
all-staff@localhost
-----------------------------< cut here >-------------------------------

Given that this is all local, I can see that this is perhaps why it
still says 'all-staff' - there's no SMTP here, for this delivery. It is
confusing, though, IMHO.

But - why is it that there are still two entries in the envelope?
--
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We believe in world peace through extortionate license fees." -Bluemeat