Short answer is that as such activity continues the incentives to compete by behaving less duplicitously also increase - that may mean unorthodox means of finding those users in extreme attack scenarios like your example tends towards - but in basic cases there are proofs nodes can provide of previous performance which can't be Sybilled.
Short answer is that as such activity continues the incentives to compete by behaving less duplicitously also increase - that may mean unorthodox means of finding those users in extreme attack scenarios like your example tends towards - but in basic cases there are proofs nodes can provide of previous performance which can't be Sybilled.