Hammer-cli parallel execution (if execute more than once at same time, vm prov. fails)

This is more a question than support request. We are using hammer to spin-up vm from templates and it works fine.

The problem we are having is: if we execute a second instance of hammer while is provisioning a vm then the second one fails to create properly. We are using libvirt as hypervisor

Is this normal?

Thanks!
Alex