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Not only can you have overlapping IP ranges in separate projects, but you can have overlapping ip ranges in the same project.
The limitation is, VMs that live on the different but identically addressed subnets will be unable to route traffic to each other by way of their internal IP address, because the VM will expect to be able to ARP and for reach the destination on its own local network subnet. This limitation also applies to VMs in different projects if you ever want to create a path between the projects by way of an RBAC shared network. This can be worked around by using FIPs on the source and destination VM for the communication path. However, that will require special consideration with regard to security groups, and will also cause the traffic to hairpin through your gateways (depending on your particular SDN setup).