locking a script

How do I lock a script so that procmail will only call that script
once (and will wait until that invocation is finished until it involves
a subsequent message)?

Here is the current call:

 this kills duplicate messages
 :0 Wh: msgid.lock
 *^(To|Cc|Bcc).*domain
   | formail -D 8192 msgid.cache
    
    :0
    *^(To|Cc|Bcc).*domain
      {
         # keep copy of all the mail caught by this rule
         :0 c
          /tmp/procmail_mail

          :0          # this is where I want the lock
          |cd /home/httpd/turn;su -c ./mail2sms.pl nobody
      }

Thank you

-- 
Eric Smith
Fruitcom.com Amsterdam
Wire phone : +31 20 528 7340
Mobile: +31 617 232 304
www.fruitcom.com
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