Bug Report

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1. Minimal reproduce step (Required)

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

3. What did you see instead (Required)

    index_modify_test.go:1084: 
        	Error Trace:	ddl/index_modify_test.go:1084
        	Error:      	"6" is not less than "6"
        	Test:       	TestAddIndexUniqueFailOnDuplicate

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/20062/tests

4. What is your TiDB version? (Required)