Why don't you comment out the line,
<property name="hibernate.hbm2ddl.auto" value="create" />
after the first deployment?
Regarding the task service from the default ant script of jbpm, that attempts to insert users and groups, you can simply handle the exceptions with try catch in the code of the class DemoTaskService.java.
Can you try with value - "validate" ?
<property name="hibernate.hbm2ddl.auto" value="validate" />
This is prevent re-creating your database schema on start up.
I have done the same and it works for me.
I know the following link is not official (i think not much official documentation exists on hibernate.hbm2ddl.auto, other that basically the excerpt found inside the link) but it is helpfull (also check out the blog mentioned in that thread),
However, whatever your decision regarding hibernate.hbm2ddl.auto , take a look at the following opinion on this matter in production environments,