Description of problem:
There is no warning or error when user tries to add new command to lockdown whitelist and this command is there already present.
This method should fail or at least warn user that this item is already in this list.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. firewall-cmd --add-lockdown-whitelist-command=/bin/bash
2. firewall-cmd --add-lockdown-whitelist-command=/bin/bash
warn or fail
This is applicable for all Command, Context, User and Uid whitelists.