lib: Ignore duplicate alist/plist entries in CLI

lib: Ignore duplicate alist/plist entries in CLI

If duplicate value is entered, the whole plist/alist just dropped.

Before:
```
$ grep prefix-list /etc/frr/frr.conf
ip prefix-list test seq 5 permit 1.1.1.1/32
ip prefix-list test seq 10 permit 1.1.1.1/32
$ systemctl restart frr
$ vtysh -c 'show run | include prefix-list'
$
```

After:
```
$ grep prefix-list /etc/frr/frr.conf
ip prefix-list test seq 5 permit 1.1.1.1/32
ip prefix-list test seq 10 permit 1.1.1.1/32
$ systemctl restart frr
$ vtysh -c 'show run | include prefix-list'
ip prefix-list test seq 5 permit 1.1.1.1/32
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>