When you want to edit a suscription ( cgi-bin/koha/serials/subscription-add.pl?op=mod&subscriptionid= ) and you click on "save subscription", then koha creates a new one instead of edits it. This is because 'modname' variable is not passed to the template in the case where op == mod.
I wrote a patch for that, it's on bug 6305. One of the problems described there is still open - item creation from serials is still broken. *** This bug has been marked as a duplicate of bug 6305 ***