Add a configuration option to allow/disallow duplicate prefixes and IPs within a VRF

[jeremystretch]

Currently, a user can create duplicate prefixes and IP addresses within a VRF (or the global table). A warning is displayed when duplicate objects are found, but no action is otherwise taken.

It should be fairly easy to implement a configuration option to disallow a user from creating duplicate prefixes or IP addresses within a VRF.

[jeremystretch]

Even better, we could make this a per-VRF toggle. And perhaps a config setting for the global table.