warn when LoadBalancer contains duplicate servers

[dfong]

a LoadBalancer should not have multiple Servers with the same name.

for example, i would like apigeelint to warn about this:

<LoadBalancer>
    <Server name="gateway-west"/>
    <Server name="gateway-west"/>
</LoadBalancer>

this is a bad practice at the very least. i would suggest making it a warning rather than an error, since neither the documentation nor the control plane forbids it. however, if this changes in the future, maybe apigeelint could upgrade it to an error.