ddl: add test for duplicate update (#6258)
ddl: add test for duplicate update (#6258)

* add test for duplicate update

wusphinx/tidb,EvilMcJerkface/tidb,CodeRushing/tidb,c4pt0r/tidb,kyle-wang/tidb,pingcap/tidb,mccxj/tidb,coocood/tidb,tiancaiamao/tidb,ngaut/tidb,c4pt0r/tidb,ngaut/tidb,EvilMcJerkface/tidb,ngaut/tidb,qiuyesuifeng/tidb,pingcap/tidb,Cofyc/tidb,qiuyesuifeng/tidb,pingcap/tidb,coocood/tidb,ngaut/tidb,kyle-wang/tidb,tiancaiamao/tidb,Nify/tidb,ming-relax/tidb,XuHuaiyu/tidb,Nify/tidb,ngaut/tidb,c4pt0r/tidb,mccxj/tidb,pingcap/tidb,kyle-wang/tidb,CodeRushing/tidb,Cofyc/tidb,cosmtrek/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,c4pt0r/tidb,kyle-wang/tidb,wusphinx/tidb,c4pt0r/tidb,ming-relax/tidb,ngaut/tidb,XuHuaiyu/tidb,cosmtrek/tidb,c4pt0r/tidb,kyle-wang/tidb