create table t (id int auto_increment, b int,  unique index (id));
insert into t values  (1,1),(1,1);
begin;
insert into t values  (1,1);
insert into t values  (1,1);

[2020/08/24 20:26:01.567 +08:00] [WARN] [session.go:476] ["can not retry txn"] [conn=1] [label=general] [error="[kv:1062]Duplicate entry '?' for key '?'"] [IsBatchInsert=false] [IsPessimistic=false] [InRestrictedSQL=false] [tidb_retry_limit=10] [tidb_disable_txn_auto_retry=true]
[2020/08/24 20:26:01.567 +08:00] [WARN] [session.go:492] ["commit failed"] [conn=1] ["finished txn"="Txn{state=invalid}"] [error="[kv:1062]Duplicate entry '?' for key '?'"]
[2020/08/24 20:26:01.567 +08:00] [ERROR] [conn.go:744] ["command dispatched failed"] [conn=1] [connInfo="id:1, addr:127.0.0.1:60651 status:10, collation:utf8_general_ci, user:root"] [command=Query] [status="inTxn:0, autocommit:1"] [sql="insert into t values ( ... )"] [txn_mode=OPTIMISTIC] [err="[kv:1062]Duplicate entry '?' for key '?'"]
[2020/08/24 20:26:01.567 +08:00] [INFO] [2pc.go:717] ["2PC clean up done"] [conn=1] [txnStartTS=418977405093019648]
[2020/08/24 20:26:01.663 +08:00] [ERROR] [conn.go:744] ["command dispatched failed"] [conn=1] [connInfo="id:1, addr:127.0.0.1:60651 status:11, collation:utf8_general_ci, user:root"] [command=Query] [status="inTxn:1, autocommit:1"] [sql="insert into t values ( ... )"] [txn_mode=OPTIMISTIC] [err="[kv:1062]Duplicate entry '?' for key '?'"]