genesis: Panic if there is a duplicate validator

[ValarDragon]

I believe that the behavior for a duplicate validator is undefined. We should panic / error if there are two identical validator addresses in the genesis file. (This is inspired by the latest testnet)

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[rigelrozanski]

I don't think we need to panic, we should definitely error and halt the genesis however

[rigelrozanski]

In a way all sorts of bad genesis is expected user input - so we should expect errors, it's not unexpected code behaviour to have two identical validators in the genesis file so shouldn't panic

[ValarDragon]

Closing in favor of #2229.