diff --git a/internal/restorable/export_test.go b/internal/restorable/export_test.go
new file mode 100644
index 000000000..533a82387
--- /dev/null
+++ b/internal/restorable/export_test.go
@@ -0,0 +1,23 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable
+
+import (
+	"image"
+)
+
+func RemoveDuplicatedRegions(regions []image.Rectangle) []image.Rectangle {
+	return removeDuplicatedRegions(regions)
+}
diff --git a/internal/restorable/rect.go b/internal/restorable/pixelrecords.go
similarity index 100%
rename from internal/restorable/rect.go
rename to internal/restorable/pixelrecords.go
diff --git a/internal/restorable/rect_test.go b/internal/restorable/rect_test.go
new file mode 100644
index 000000000..494f59573
--- /dev/null
+++ b/internal/restorable/rect_test.go
@@ -0,0 +1,131 @@
+// Copyright 2023 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package restorable_test
+
+import (
+	"image"
+	"testing"
+
+	"github.com/hajimehoshi/ebiten/v2/internal/restorable"
+)
+
+func areEqualRectangles(a, b []image.Rectangle) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+
+	return true
+}
+
+func TestRemoveDuplicatedRegions(t *testing.T) {
+	cases := []struct {
+		In  []image.Rectangle
+		Out []image.Rectangle
+	}{
+		{
+			In:  nil,
+			Out: nil,
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 2, 2),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 2, 2),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 1, 1),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 1, 1),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 1, 1),
+				image.Rect(0, 0, 2, 2),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 2, 2),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 1, 3),
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 3, 1),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 1, 3),
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 3, 1),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 1, 3),
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 3, 1),
+				image.Rect(0, 0, 4, 4),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 4, 4),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 1, 3),
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 3, 1),
+				image.Rect(0, 0, 4, 4),
+				image.Rect(1, 1, 2, 2),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 4, 4),
+				image.Rect(1, 1, 2, 2),
+			},
+		},
+		{
+			In: []image.Rectangle{
+				image.Rect(0, 0, 1, 3),
+				image.Rect(0, 0, 2, 2),
+				image.Rect(0, 0, 3, 1),
+				image.Rect(0, 0, 4, 4),
+				image.Rect(0, 0, 5, 5),
+			},
+			Out: []image.Rectangle{
+				image.Rect(0, 0, 5, 5),
+			},
+		},
+	}
+
+	for _, c := range cases {
+		got := restorable.RemoveDuplicatedRegions(c.In)
+		want := c.Out
+		if !areEqualRectangles(got, want) {
+			t.Errorf("restorable.RemoveDuplicatedRegions(%#v): got: %#v, want: %#v", c.In, got, want)
+		}
+	}
+}