[Zope] Double precondition with PDF files

hi i'm experiencing a problem with preconditions. í use the precondition of files to track the downloading of these. if i upload ppt's or doc's there aren't any problems. when i download i get one line in my logfile. if i download a pdf then the log shows 2 entries. to judge from the time the first is when the download starts an the secon when it ends. anybod got an idea on why this is happening? oh...zope 2.4.0 & python 2.1.1 (I know it's old! but updating that is difficult) thanx for the help! oliver erlewein