issue when adding static IPv6 to interface, but IP exists as virtual

[CanadaGuy]

I was running into issues with radvd not starting, after I made some IPv6 changes. After debug I discovered that there will be no error or warning presented when attempting to set a static IPv6 address that already exists as a virtual IP (on the same interface).

I accept this is 100% user error, but perhaps checking the virtual IP list when adding an interface static IP could prevent similar issues. I was switching between IPv6 subnets, so I had my new subnet IP as a virtual IP while I was attempting to debug other connectivity issues.