The HTable.increment() operation is non-idempotent. The client retries the increment RPC a few times (as specified by configuration) before throwing an error to the application. This makes it possible that the same increment call be applied twice at the server.
For increment operations, is it better to use HConnectionManager.getRegionServerWithoutRetries()? Another option would be to enhance the IPC module to make the RPC server correctly identify if the RPC is a retry attempt and handle accordingly.