Starting mongod: error command line: multiple occurrences

I tried to start mongod service and got error:

[root@mongo root]# service mongod start
Starting mongod: error command line: multiple occurrences
use --help for help
                                                           [  OK  ]

service didn't start an didn't log nothing
problem was into /etc/mongod.conf
I set one of parameters double into config, in my case it was "fork = true"
after removed one, service started successfully