obsolete: do not accept duplicated marker during exchange
Before this patch, duplicated obsolescence markers could slip into an
obstore if the bookmark was unknown locally and duplicated in the
incoming obsolescence stream.
Existing duplicate markers will not be automatically removed but
they'll stop propagating. Having a few duplicated markers is harmless
and people have been warned evolution is <blink>experimental</blink>
anyway.
(binary:application/octet-stream)