[BUG] ignore two delete case for seq set violation

src/storage/memtable/ob_concurrent_control.cpp

@@ -104,8 +104,18 @@ int check_sequence_set_violation(const concurrent_control::ObWriteFlag write_fla
     // and so fail to pass the check. We must bypass the case.
     } else if (write_flag.is_table_api()) {
       // bypass the case
+    // Case 6: For the case of deleting rows during building the unique index
+    // concurrently, it may exist that two rows of the main table point to one
+    // row of the newly created index, which means the unique index will abort
+    // itself during consistency check. While because of the feature of the
+    // online ddl, the concurrent delete will start to operate on the newly
+    // created index, which causes these two delete operations and fail to pass
+    // the check. So we need bypass this case.
+    } else if (blocksstable::ObDmlFlag::DF_DELETE == writer_dml_flag
+               && blocksstable::ObDmlFlag::DF_DELETE == locker_dml_flag) {
+      // bypass the case
     } else {
-    // Case 6: It will never happen that two operaions on the same row for the
+    // Case 7: It will never happen that two operaions on the same row for the
     // same txn except the above cases. So we should report unexpected error.
       ret = OB_ERR_UNEXPECTED;
       TRANS_LOG(ERROR, "multiple modification on one row found", K(reader_seq_no),