adding unique index reports "duplicate entry" unexpectedly

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Start a workload that contains DMLs(like REPLACE INTO) on the target table.
2. Add a unique index.

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

No error.

3. What did you see instead (Required)

Duplicate entry '?' for key 'idx'

4. What is your TiDB version? (Required)

c13eb90