duplicate option doesn't work with a list of object to print

See below the steps to reproduce the issue :

1. create an integer field on partner's form (for example)
2. in the jasper document form, fill the field 'duplicate' with the field you have created before "o.partner_id.number_of_copy" for example
3. in tree view, select more than one record and click on "Print > My Document"

result :
the number of copy of the last record will be applied to all the records.

if the first partner has a number of copy equals to 2 and the last partner has a number of copy equals to 3, then 3 copies of the report for the first partner will be done, 3 for the second, ...., 3 for the last one