diff --git a/pkg/keyspace/tso_keyspace_group.go b/pkg/keyspace/tso_keyspace_group.go
index 663b7835487..9b05aac0d41 100644
--- a/pkg/keyspace/tso_keyspace_group.go
+++ b/pkg/keyspace/tso_keyspace_group.go
@@ -616,7 +616,17 @@ func buildSplitKeyspaces(
 				oldSplit = append(oldSplit, keyspace)
 			}
 		}
-		return oldSplit, new, nil
+		// If newNum != len(newKeyspaceMap), it means the provided new keyspace list contains
+		// duplicate keyspaces, and we need to dedup them (https://github.com/tikv/pd/issues/6687);
+		// otherwise, we can just return the old split and new keyspace list.
+		if newNum == len(newKeyspaceMap) {
+			return oldSplit, new, nil
+		}
+		newSplit := make([]uint32, 0, len(newKeyspaceMap))
+		for keyspace := range newKeyspaceMap {
+			newSplit = append(newSplit, keyspace)
+		}
+		return oldSplit, newSplit, nil
 	}
 	// Split according to the start and end keyspace ID.
 	if startKeyspaceID == 0 && endKeyspaceID == 0 {
@@ -628,7 +638,9 @@ func buildSplitKeyspaces(
 	)
 	for _, keyspace := range old {
 		if keyspace == utils.DefaultKeyspaceID {
-			return nil, nil, ErrModifyDefaultKeyspace
+			// The source keyspace group must be the default keyspace group and we always keep the default
+			// keyspace in the default keyspace group.
+			continue
 		}
 		if startKeyspaceID <= keyspace && keyspace <= endKeyspaceID {
 			newSplit = append(newSplit, keyspace)
diff --git a/pkg/keyspace/tso_keyspace_group_test.go b/pkg/keyspace/tso_keyspace_group_test.go
index 40b779382cd..e8a40a839c8 100644
--- a/pkg/keyspace/tso_keyspace_group_test.go
+++ b/pkg/keyspace/tso_keyspace_group_test.go
@@ -483,6 +483,26 @@ func TestBuildSplitKeyspaces(t *testing.T) {
 			new: []uint32{6},
 			err: ErrKeyspaceNotInKeyspaceGroup,
 		},
+		{
+			old:         []uint32{1, 2},
+			new:         []uint32{2, 2},
+			expectedOld: []uint32{1},
+			expectedNew: []uint32{2},
+		},
+		{
+			old:             []uint32{0, 1, 2, 3, 4, 5},
+			startKeyspaceID: 2,
+			endKeyspaceID:   4,
+			expectedOld:     []uint32{0, 1, 5},
+			expectedNew:     []uint32{2, 3, 4},
+		},
+		{
+			old:             []uint32{0, 1, 2, 3, 4, 5},
+			startKeyspaceID: 0,
+			endKeyspaceID:   4,
+			expectedOld:     []uint32{0, 5},
+			expectedNew:     []uint32{1, 2, 3, 4},
+		},
 		{
 			old:             []uint32{1, 2, 3, 4, 5},
 			startKeyspaceID: 2,
@@ -490,6 +510,13 @@ func TestBuildSplitKeyspaces(t *testing.T) {
 			expectedOld:     []uint32{1, 5},
 			expectedNew:     []uint32{2, 3, 4},
 		},
+		{
+			old:             []uint32{1, 2, 3, 4, 5},
+			startKeyspaceID: 5,
+			endKeyspaceID:   6,
+			expectedOld:     []uint32{1, 2, 3, 4},
+			expectedNew:     []uint32{5},
+		},
 		{
 			old:             []uint32{1, 2, 3, 4, 5},
 			startKeyspaceID: 2,
@@ -497,6 +524,13 @@ func TestBuildSplitKeyspaces(t *testing.T) {
 			expectedOld:     []uint32{1},
 			expectedNew:     []uint32{2, 3, 4, 5},
 		},
+		{
+			old:             []uint32{1, 2, 3, 4, 5},
+			startKeyspaceID: 1,
+			endKeyspaceID:   1,
+			expectedOld:     []uint32{2, 3, 4, 5},
+			expectedNew:     []uint32{1},
+		},
 		{
 			old:             []uint32{1, 2, 3, 4, 5},
 			startKeyspaceID: 0,
@@ -504,6 +538,13 @@ func TestBuildSplitKeyspaces(t *testing.T) {
 			expectedOld:     []uint32{},
 			expectedNew:     []uint32{1, 2, 3, 4, 5},
 		},
+		{
+			old:             []uint32{1, 2, 3, 4, 5},
+			startKeyspaceID: 7,
+			endKeyspaceID:   10,
+			expectedOld:     []uint32{1, 2, 3, 4, 5},
+			expectedNew:     []uint32{},
+		},
 		{
 			old: []uint32{1, 2, 3, 4, 5},
 			err: ErrKeyspaceNotInKeyspaceGroup,