[c-nsp] IOS-XR accepted duplicate subnet configurations for interface

[Christoffer Hansen]

Dear c-nsp fellows,

I am not sure if any one of you would have an answer the the below
example...

I have recently run into a case with an ASR9k router running IOS-XR
v5.3.4. Were I by accident put an identical secondary subnet on a 2nd
interface located inside the same VRF as the first one. It is even a 2nd
sub-interface to another sub-interface on the same main interface.

Case-in-point: The router accepted the configuration commit without
complaints and of course traffic then stops flowing.

Normally I would not expect this to be possible to do. And would expect
the router to output a warning telling me I am trying to commit an IPv4
address|subnet already configured on another interface in the same VRF.


Q: Would you expect
(1) a warning in my scenario or
(2) the router just accepting the staged configuration change upon commit?


```iosxr
!!! 1st-subinterface
!
interface GigabitEthernet666/0/0/2.1478 !!nvSatellite interface
 vrf ROUTING-INSTANCE-INTERNET
 ipv4 mtu 1500
 ipv4 address 198.51.100.1 255.255.255.252
 ipv4 address 203.0.113.1 255.255.255.252 secondary
 ipv4 verify unicast source reachable-via any allow-default
 encapsulation dot1q 1478
!

!!! 2nd sub-interface
!
interface GigabitEthernet666/0/0/2.665 !!nvSatellite interface
 vrf ROUTING-INSTANCE-INTERNET
 ipv4 mtu 1500
 ipv4 address 192.0.2.1 255.255.255.252
 ipv4 address 203.0.113.1 255.255.255.252 secondary !!committed line
 ipv4 verify unicast source reachable-via any allow-default
 encapsulation dot1q 665
!
```

-Christoffer
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