The current implementation of claimTokens method blacklists a pubkeyID for given withdrawRoot to prevent claiming the same token amount twice.

As a result this introduces a restriction on the MassMigration transactions that can be put into a single commitment. Namely, there can be no two transactions from UserStates with the same pubkeyID in a single MassMigration commitment.

Instead we could blacklist a specific path in withdraw states tree to drop this unnecessary restriction.