When checking for duplication the IP address assigned to subscriber, there is a need to check also whether these IPs are in the same VRF. Otherwise you block the ability for overlapping address space in different VRFs. We've got a customer who has up to 500 duplicated (and even triplicated) addresses in different VRFs and have to set check-ip=0. Actually this is bad too because we're not protected from provisioning mistakes within the particular VRF, but it's not that evil as no service for 500 subs.
I did not check with more recent versions, so correct me if that's already been fixed later.