Multiple SpamAssassins: how many X-Spam-Status header fields?

Hi,

An email travelling through multiple MTAs at different institutions
arrives with the earlier SpamAssassin X-Spam-... header fields intact,
including X-Spam-Status, but a later check adds X-Spam-... fields except
for X-Spam-Status.

The later institution say this is because only one X-Spam-Status should
be present so they don't add a second, or replace the first.  It means
I see an X-Spam-Flag: No from the later check but not the tests which
hit.

- Is there some documentation for me to read which says one
  X-Spam-Status is the maximum?

- If one is the maximum, which of the assassins has their field kept?

- Where is SpamAssassin's logic to handle this?

I've had a look at the source — add_header, ham_headers, etc. —
but don't obviously see anything which avoids adding a duplicate.

-- 
Thanks, Ralph.