as I configured Postfix to use spamassassin and Dovecot LDA (Local Delivery Agent), everything works fine but I got in the Postfix log two steps 'delivered via'.

Questions :
is it correct ?
would it be better to have only one 'delivered via' for performance reason or other ?
should Dovecot LDA call spamassassin itself ?

Thanks for your attention and advice.

* config
smtp inet n - - - - smtpd
-o content_filter=myspamd

dovecot unix - n n - - pipe
flags=DRhu user=vmail argv=/usr/lib/dovecot/deliver -f ${sender} -d ${user}@${nexthop} -a ${recipient} -n -m ${extension}

myspamd unix - n n - - pipe
user=spamfilter argv=/usr/bin/spamc -f
-e /usr/sbin/sendmail -oi -f ${sender} ${recipient}

* config
virtual_transport = dovecot

* Postfix log:
postfix/qmgr[16113]: 1A6B13574185: from=<m-UJtVjqJKRiNK03qxhMDCKkDl4jUge64gthypWki3RiZCW7pRtV>, size=90054, nrcpt=1 (queue active)
spamd[11764]: spamd: connection from localhost [] at port 45021
spamd[11764]: spamd: setuid to spamfilter succeeded
spamd[11764]: spamd: processing message <> for spamfilter:5001
postfix/smtpd[5920]: disconnect from[]
spamd[11764]: spamd: clean message (-6.9/2.0) for spamfilter:5001 in 12.5 seconds, 88533 bytes.
spamd[11764]: spamd: result: . -6 - BAYES_00,HTML_MESSAGE,RCVD_IN_RP_CERTIFIED,RCVD_IN _RP_SAFE,T_DKIM_INVALID scantime=12.5,size=88533,user=spamfilter,uid=5001, required_score=2.0,rhost=localhost,raddr= ,rport=45021,mid=<53198259.19553386.1355330534670.>,bayes=0.000000,autolearn=ham
spamd[17643]: prefork: child states: I
postfix/pickup[1863]: F2C6B54D2008: uid=5e001 from=<m-UJtVjqJKRiNK03qxhMDCKkDl4jUge64gthypWki3RiZCW7pRtV>
postfix/cleanup[5923]: F2C6B54D2008: message-id=<>
postfix/pipe[5924]: 1A6B13574185: to=<>, relay=myspamd, delay=14, delays=1.5/0.01/0/13, dsn=2.0.0, status=sent (delivered via myspamd service)
postfix/qmgr[16113]: 1A6B13574185: removed
postfix/qmgr[16113]: F2C6B54D2008: from=<m-UJtVjqJKRiNK03qxhMDCKkDl4jUge64gthypWki3RiZCW7pRtV>, size=90434, nrcpt=1 (queue active)
dovecot: deliver( sieve: msgid=<53198259.19553386.1355330534670.JavaMail.ap>: stored mail into mailbox 'INBOX'
postfix/pipe[5928]: F2C6B54D2008: to=<>, relay=dovecot, delay=0.58, delays=0.43/0/0/0.15, dsn=2.0.0, status=sent (delivered via dovecot service)
postfix/qmgr[16113]: F2C6B54D2008: removed