为提高效率，提问时请提供以下信息，问题描述清晰可优先响应。
- 【TiDB 版本】：v3.0.9
- 【DM版本】：v1.0.3
- 【问题描述】：
分库分表场景下，上游库中存的是普通索引，下游建了唯一索引，且dm同步完成，应要求，此时需要对上游普通 索引进行清理重复数据，增加了唯一索引 ，删除了普通索引，对于这种情况如何处理？
简单说就是上游增加的唯一索引现在在tidb中已经存在，现在ddl过不去了，导致同步延迟严重，如何处理？
【报错内容】：[2020/03/06 10:08:01.939 +08:00] [INFO] [server.go:307] [“receive DDLLockInfo”] [request=FetchDDLInfo] [“ddl lock info”=“task:“product_all_merge” ID:“product_all_merge-
product_x.item_props_x” “] [2020/03/06 10:08:01.939 +08:00] [ERROR] [server.go:315] [“fail to record DDLLockInfo”] [request=FetchDDLInfo] [“ddl lock info”=“task:“product_all_merge” ID:“product_all_merge-product_x.item_props_x” “] [error=”[code=40062:class=dm-worker:scope=internal:level=high] DDLLockInfo for task product_all_merge already exists”] [errorVerbose=”[code=40062:class=dm-worker:scope=internal:level=high] DDLLockInfo for task product_all_merge already exists\ngithub.com/pingcap/dm/pkg/terror.(*Error).Generate\ \t/go/src/github.com/pingcap/dm/pkg/terror/terror.go:232\ngithub.com/pingcap/dm/dm/worker.(*SubTask).SaveDDLLockInfo\ \t/go/src/github.com/pingcap/dm/dm/worker/subtask.go:529\ngithub.com/pingcap/dm/dm/worker.(*Worker).RecordDDLLockInfo\ \t/go/src/github.com/pingcap/dm/dm/worker/worker.go:445\ngithub.com/pingcap/dm/dm/worker.(*Server).FetchDDLInfo\ \t/go/src/github.com/pingcap/dm/dm/worker/server.go:311\ngithub.com/pingcap/dm/dm/pb._Worker_FetchDDLInfo_Handler\ \t/go/src/github.com/pingcap/dm/dm/pb/dmworker.pb.go:3865\ google.golang.org/grpc.(*Server).processStreamingRPC\ \t/go/pkg/mod/google.golang.org/grpc@v1.25.1/server.go:1211\ngoogle.golang.org/grpc.(*Server).handleStream\ \t/go/pkg/mod/google.golang.org/grpc@v1.25.1/server.go:1291\ngoogle.golang.org/grpc.(*Server).serveStreams.func1.1\ \t/go/pkg/mod/google.golang.org/grpc@v1.25.1/server.go:722\ runtime.goexit\ \t/usr/local/go/src/runtime/asm_amd64.s:1357”]
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