The duplicated code can be as simple as a few cryptographic operations for each broadcasted transaction. This can easily scale to tens of thousands of nodes and transactions per hour without requiring significant computing resources over the baseline of running a high bandwidth internet node.

When the consistency algorithm is proof of work chains, or when the proof of work is used to control the scarcity and disbursement of the valuable token, only then you have the whole "world turning into fucking Mordor" problem.