lockingSystem Concurrency problem

I am running vCO 5.5.1.

I have a workflow that allocates an IP address which are stored in configuration items. Kind of like a home gronw IPAM system. To avoid handing out double ip addresses I am using the lockingSystem of vRO.

But here is the problem: When two workflows request the lock at exactly the same time they both get the lock and I end up with duplicate IP addresses.

In my opinion this should never happen. Is this a known problem? Anything I can to to avoid thig other than adding random delay at the start of the script?