Sybil Attack : Bitcoin, Ethereum, blockchain, Cryptocurrency

By definition:

A Sybil attack occurs when one actor acts as multiple separate entities. Because many distributed systems have no form of identity management beyond accounts, and because accounts are trivially created, any actor can create an unbounded number of accounts.

Avoiding Sybil attacks is difficult. In centralized systems it can be done through heuristics that do not provide cryptographic assurance of Sybil resilience. For example, a centralized entity may try to avoid Sybil attacks by requiring that an individual IP cannot create more than a specific number of user accounts in a given time interval.

Sybil attacks are avoided in Bitcoin by requiring block generation ability to be proportional to computational power available through the proof-of-work mechanism. That way, an adversary is limited in how many blocks they can produce. This provides strong cryptographic guarantees of Sybil resilience.

For Ethereum and DAPP networks it is a bit harder, for instance, you want to implement a voting system, or in other situations where who someone is, or whether two different identifies refer to the same person matters, such as an auction (where allowing the seller to bid on their own items would give them an unfair advantage).