To test, either run 'play test' (which times out..) or 'play run' and then do the following curl twice (the second time should not return OK):
curl -i -X POST http://localhost:9000/users
You will need the play 2.1 snapshot
Monitor the mongodb collection 'users' in the database 'test'.
use test db.users.find()
Also check that the unique index for 'email' is created ok.