As North made his final pass, West said something along the lines of: "Oh my partner's double was for penalties. It can't be anything else, I probably should have alerted it shouldn't I?" Penalties is indeed the partnership understanding.
North agrees, TD is called and North has no desire to change his call.
Result 3♥X-3 NS -800.
NS claim damage as South would bid 4♣ over an alerted double. How would you rule?

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1♠-3♥-X-AP