diff --git a/ddl/index.go b/ddl/index.go
index 478338a78adc5..3a25f49e6a257 100644
--- a/ddl/index.go
+++ b/ddl/index.go
@@ -1585,10 +1585,18 @@ func (w *addIndexWorker) BackfillDataInTxn(handleRange reorgBackfillTask) (taskC
 		return nil
 	})
 	logSlowOperations(time.Since(oprStartTime), "AddIndexBackfillDataInTxn", 3000)
-
+	failpoint.Inject("mockDMLExecution", func(val failpoint.Value) {
+		//nolint:forcetypeassert
+		if val.(bool) && MockDMLExecution != nil {
+			MockDMLExecution()
+		}
+	})
 	return
 }
 
+// MockDMLExecution is only used for test.
+var MockDMLExecution func()
+
 func (w *worker) addPhysicalTableIndex(t table.PhysicalTable, reorgInfo *reorgInfo) error {
 	if reorgInfo.mergingTmpIdx {
 		logutil.BgLogger().Info("[ddl] start to merge temp index", zap.String("job", reorgInfo.Job.String()), zap.String("reorgInfo", reorgInfo.String()))
diff --git a/ddl/index_merge_tmp.go b/ddl/index_merge_tmp.go
index a7b9816fc85de..2cf3b6d1d106d 100644
--- a/ddl/index_merge_tmp.go
+++ b/ddl/index_merge_tmp.go
@@ -49,25 +49,12 @@ func (w *mergeIndexWorker) batchCheckTemporaryUniqueKey(txn kv.Transaction, idxR
 		return errors.Trace(err)
 	}
 
-	// 1. unique-key/primary-key is duplicate and the handle is equal, skip it.
-	// 2. unique-key/primary-key is duplicate and the handle is not equal, return duplicate error.
-	// 3. non-unique-key is duplicate, skip it.
 	for i, key := range w.originIdxKeys {
 		if val, found := batchVals[string(key)]; found {
-			if idxRecords[i].distinct && !bytes.Equal(val, idxRecords[i].vals) {
-				return kv.ErrKeyExists
-			}
-			if !idxRecords[i].delete {
-				idxRecords[i].skip = true
-			} else {
-				// Prevent deleting an unexpected index KV.
-				hdInVal, err := tablecodec.DecodeHandleInUniqueIndexValue(val, w.table.Meta().IsCommonHandle)
-				if err != nil {
-					return errors.Trace(err)
-				}
-				if !idxRecords[i].handle.Equal(hdInVal) {
-					idxRecords[i].skip = true
-				}
+			// Found a value in the original index key.
+			err := checkTempIndexKey(txn, idxRecords[i], val, w.table)
+			if err != nil {
+				return errors.Trace(err)
 			}
 		} else if idxRecords[i].distinct {
 			// The keys in w.batchCheckKeys also maybe duplicate,
@@ -78,6 +65,48 @@ func (w *mergeIndexWorker) batchCheckTemporaryUniqueKey(txn kv.Transaction, idxR
 	return nil
 }
 
+func checkTempIndexKey(txn kv.Transaction, tmpRec *temporaryIndexRecord, originIdxVal []byte, tblInfo table.Table) error {
+	if !tmpRec.delete {
+		if tmpRec.distinct && !bytes.Equal(originIdxVal, tmpRec.vals) {
+			return kv.ErrKeyExists
+		}
+		// The key has been found in the original index, skip merging it.
+		tmpRec.skip = true
+		return nil
+	}
+	// Delete operation.
+	distinct := tablecodec.IndexKVIsUnique(originIdxVal)
+	if !distinct {
+		// For non-distinct key, it is consist of a null value and the handle.
+		// Same as the non-unique indexes, replay the delete operation on non-distinct keys.
+		return nil
+	}
+	// For distinct index key values, prevent deleting an unexpected index KV in original index.
+	hdInVal, err := tablecodec.DecodeHandleInUniqueIndexValue(originIdxVal, tblInfo.Meta().IsCommonHandle)
+	if err != nil {
+		return errors.Trace(err)
+	}
+	if !tmpRec.handle.Equal(hdInVal) {
+		// The inequality means multiple modifications happened in the same key.
+		// We use the handle in origin index value to check if the row exists.
+		rowKey := tablecodec.EncodeRecordKey(tblInfo.RecordPrefix(), hdInVal)
+		_, err := txn.Get(context.Background(), rowKey)
+		if err != nil {
+			if kv.IsErrNotFound(err) {
+				// The row is deleted, so we can merge the delete operation to the origin index.
+				tmpRec.skip = false
+				return nil
+			}
+			// Unexpected errors.
+			return errors.Trace(err)
+		}
+		// Don't delete the index key if the row exists.
+		tmpRec.skip = true
+		return nil
+	}
+	return nil
+}
+
 // temporaryIndexRecord is the record information of an index.
 type temporaryIndexRecord struct {
 	vals     []byte
diff --git a/ddl/index_merge_tmp_test.go b/ddl/index_merge_tmp_test.go
index 1c4d107a0aebf..563b8614d4311 100644
--- a/ddl/index_merge_tmp_test.go
+++ b/ddl/index_merge_tmp_test.go
@@ -18,6 +18,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/pingcap/failpoint"
 	"github.com/pingcap/tidb/ddl"
 	"github.com/pingcap/tidb/ddl/ingest"
 	"github.com/pingcap/tidb/domain"
@@ -415,6 +416,70 @@ func TestAddIndexMergeDeleteUniqueOnWriteOnly(t *testing.T) {
 	tk.MustExec("admin check table t;")
 }
 
+func TestAddIndexMergeDeleteNullUnique(t *testing.T) {
+	store := testkit.CreateMockStore(t)
+
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("create table t(id int primary key, a int default 0);")
+	tk.MustExec("insert into t values (1, 1), (2, null);")
+
+	tk1 := testkit.NewTestKit(t, store)
+	tk1.MustExec("use test")
+
+	ddl.MockDMLExecution = func() {
+		_, err := tk1.Exec("delete from t where id = 2;")
+		assert.NoError(t, err)
+	}
+	require.NoError(t, failpoint.Enable("github.com/pingcap/tidb/ddl/mockDMLExecution", "1*return(true)->return(false)"))
+	tk.MustExec("alter table t add unique index idx(a);")
+	tk.MustQuery("select count(1) from t;").Check(testkit.Rows("1"))
+	tk.MustExec("admin check table t;")
+	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/ddl/mockDMLExecution"))
+}
+
+func TestAddIndexMergeDoubleDelete(t *testing.T) {
+	store, dom := testkit.CreateMockStoreAndDomain(t)
+
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+	tk.MustExec("create table t(id int primary key, a int default 0);")
+
+	tk1 := testkit.NewTestKit(t, store)
+	tk1.MustExec("use test")
+
+	d := dom.DDL()
+	originalCallback := d.GetHook()
+	defer d.SetHook(originalCallback)
+	callback := &ddl.TestDDLCallback{}
+	onJobUpdatedExportedFunc := func(job *model.Job) {
+		if t.Failed() {
+			return
+		}
+		switch job.SchemaState {
+		case model.StateWriteOnly:
+			_, err := tk1.Exec("insert into t values (1, 1);")
+			assert.NoError(t, err)
+		}
+	}
+	callback.OnJobUpdatedExported.Store(&onJobUpdatedExportedFunc)
+	d.SetHook(callback)
+
+	ddl.MockDMLExecution = func() {
+		_, err := tk1.Exec("delete from t where id = 1;")
+		assert.NoError(t, err)
+		_, err = tk1.Exec("insert into t values (2, 1);")
+		assert.NoError(t, err)
+		_, err = tk1.Exec("delete from t where id = 2;")
+		assert.NoError(t, err)
+	}
+	require.NoError(t, failpoint.Enable("github.com/pingcap/tidb/ddl/mockDMLExecution", "1*return(true)->return(false)"))
+	tk.MustExec("alter table t add unique index idx(a);")
+	tk.MustQuery("select count(1) from t;").Check(testkit.Rows("0"))
+	tk.MustExec("admin check table t;")
+	require.NoError(t, failpoint.Disable("github.com/pingcap/tidb/ddl/mockDMLExecution"))
+}
+
 func TestAddIndexMergeConflictWithPessimistic(t *testing.T) {
 	store, dom := testkit.CreateMockStoreAndDomain(t)
 	tk := testkit.NewTestKit(t, store)