[FIX] ir_qweb: handle duplicated preprocessed assets
[FIX] ir_qweb: handle duplicated preprocessed assets

Previous implementation was expecting exactly the same number of
attachments than the number of assets in a bundle. However it's
possible that duplicated assets are created due to concurrent first
generation.

This duplication cannot be avoid, but is cleaned upon next bundle
regeneration.

Joint work with @sle-odoo and @odony

laslabs/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,hip-odoo/odoo,stephen144/odoo,laslabs/odoo,microcom/odoo,bplancher/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,hip-odoo/odoo,ygol/odoo,dfang/odoo,stephen144/odoo,storm-computers/odoo,microcom/odoo,dfang/odoo,storm-computers/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,stephen144/odoo,hip-odoo/odoo,storm-computers/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,storm-computers/odoo,ygol/odoo,ygol/odoo,dfang/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,bplancher/odoo,microcom/odoo,bplancher/odoo,ygol/odoo,laslabs/odoo,bplancher/odoo,stephen144/odoo,hip-odoo/odoo,stephen144/odoo,sysadminmatmoz/OCB,storm-computers/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,stephen144/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,ygol/odoo,storm-computers/odoo,bplancher/odoo,ygol/odoo,laslabs/odoo,microcom/odoo,dfang/odoo,microcom/odoo,microcom/odoo,dfang/odoo