Can the same address be added as a verification more than once on the protocol today? If so, when a verification remove message is broadcasted, does Hubble clean up all previous add verification messages of the same address or just 1 of them?

FIP-9 introduced globally unique verifications

https://github.com/farcasterxyz/protocol/discussions/114

added to Hubble here

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FIP-9 introduced globally unique verifications

https://github.com/farcasterxyz/protocol/discussions/114

added to Hubble here 
https://github.com/farcasterxyz/hub-monorepo/pull/1450