Is it possible for duplicates to appear during replication? Can a user have two different balances?

The appearance of duplicates during replication depends on the type of replication and the system architecture. In classic asynchronous replication, a situation may occur where, due to delays or failures, data on replicas temporarily diverges, which can lead to inconsistency, such as different balances for the same user on different nodes.

To prevent the appearance of two different balances, the following are usually applied:

• Idempotent operations and unique keys to prevent transaction duplication.
• Use of consensus protocols (e.g., Raft, Paxos) for state agreement.
• Application of transactions and locks at the database level.
• Use of systems supporting strong consistency.

Example: if a user's balance is updated on the master node, and the replica is updated with a delay, then during the delay, the balance on the replica may differ. But with proper replication setup and conflict handling, the data ultimately synchronizes and duplicates do not appear.