ejabberdctl concurrent option usage

Dear all,

I'm using ejabberd-2.1.10 and I have created a java command line to run ejabberdctl srg-user-add.
This program run nicely when it is on a single call, but when I'm trying to run it in a concurrent thread, the command line returns "duplicate_name" error.
I've search around on the internet and see there is an option to run ejabberdctl in concurrent mode.
Does anyone know how to do that?? I've tried to run ejabberdctl --concurrent srg-user-add but with no luck.
Thanks in advance.

Jeffery