DataGen (number) not unique when burst testing

In a TestCase, I use DataGen with one Number Type generated property (inc_number) with:
Start: 100
End:  999999
Step:  1
and "Persist series between test runs"

I use that number in several TestCase Step requests to form unique email addresses like this:
${DataGen#inc_number}@soapui.local

Now when I run a Burst LoadTest with 30 Threads, Burst Delay 30 and Duration 60, I receive faults from my application, that one tried to create an entry with a non unique email address.
I get this in my app logs:

2009-10-28 15:02:32,695 [TP-Processor12] DEBUG xxx - Creating user >2094@soapui.local<
2009-10-28 15:02:32,697 [TP-Processor30] DEBUG xxx - Creating user >2094@soapui.local<
So it seems SoapUI tries to use the same generated value at the same time in two different test step threads.
Next request is with 206@soapui.local, so it seems it used 2094 two times, but not 2095.

This happens after approx. 4 Minutes of running the load test.
During this time, SoapUI already created hundreds of entries successfully in my app.

Is there a known problem with DataGen when machine is under heavy load?
I'm on MacOS Snow Leopard 10.6.1.