insert ignore on duplicate got wrong result when index value unchanged

[lysu]

Bug Report

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1. Minimal reproduce step (Required)

create table t6 (a int, b int, c int, primary key(a, b) clustered, unique key idx_14(b), unique key idx_15(b), unique key idx_16(a, b));
insert into t6 select 10, 10, 20;
insert ignore into t6 set a = 20, b = 10 on duplicate key update a = 100;

2. What did you expect to see? (Required)

on duplicate on b=10 and update a to 100

3. What did you see instead (Required)

update nothing

4. What is your TiDB version? (Required)

master

[lysu]

introduced by #23814

only in master, not affect on 5.0..

[ti-srebot]

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