diff --git a/cmd/config.go b/cmd/config.go
index 4385a50..4588b04 100644
--- a/cmd/config.go
+++ b/cmd/config.go
@@ -22,6 +22,7 @@ type config struct {
 	addTags     string
 	dryRun      bool
 	archive     string
+	dedupe      bool
 }
 
 func getConfig() config {
@@ -39,6 +40,7 @@ func getConfig() config {
 	flag.StringVar(&tagss, "tags", "", "csv of all tags to find, -x to invert")
 	flag.StringVar(&config.search, "search", "", "fts case insensitive")
 	flag.BoolVar(&config.edit, "e", false, "edit file")
+	flag.BoolVar(&config.dedupe, "dedupe", false, "dedupe file")
 	flag.BoolVar(&config.dryRun, "dry-run", false, "dry run")
 	flag.StringVar(&config.add, "add", "", "todo to add")
 	flag.StringVar(&config.addSchedule, "add-schedule", "", "todo to add schedule")
diff --git a/cmd/go.mod b/cmd/go.mod
index b82262e..e2edeff 100644
--- a/cmd/go.mod
+++ b/cmd/go.mod
@@ -1,6 +1,6 @@
 module pttodo-cli
 
-go 1.17
+go 1.21
 
 require (
 	gitea.inhome.blapointe.com/gogs/pttodo v0.0.0-20231109151914-5d74b458d542
diff --git a/cmd/main.go b/cmd/main.go
index 3008245..cd85b4e 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -7,6 +7,8 @@ import (
 	"log"
 	"os"
 
+	"slices"
+
 	"gitea.inhome.blapointe.com/gogs/pttodo/pttodo"
 
 	"gopkg.in/yaml.v2"
@@ -32,6 +34,8 @@ func _main() error {
 		return merge(&config)
 	} else if config.archive != "" {
 		return archive(&config)
+	} else if config.dedupe {
+		return dedupe(&config)
 	} else {
 		if err := add(&config); err != nil {
 			return err
@@ -43,6 +47,54 @@ func _main() error {
 	}
 }
 
+func dedupe(config *config) error {
+	baseReader, err := filePathReader(config.target)
+	if err != nil {
+		return err
+	}
+	baseB, err := ioutil.ReadAll(baseReader)
+	if err != nil {
+		return err
+	}
+
+	var base pttodo.Root
+	if err := yaml.Unmarshal(baseB, &base); err != nil {
+		return err
+	}
+
+	do := func(todos []pttodo.Todo) []pttodo.Todo {
+		i := len(todos) - 1
+		for i >= 0 {
+			j := slices.IndexFunc(todos, func(todo pttodo.Todo) bool {
+				return todo.Equals(todos[i])
+			})
+			if j >= 0 && j != i {
+				todos[j] = todos[i]
+				base.Done = append(base.Done, todos[j])
+				todos = todos[:i]
+			}
+			i -= 1
+		}
+		return todos
+	}
+
+	base.Todo = do(base.Todo)
+	base.Scheduled = do(base.Scheduled)
+
+	if tmppath, err := marshalRootToTempFile(base); err != nil {
+		return err
+	} else if config.dryRun {
+		log.Println("===before===")
+		_dump(os.Stdout, []string{config.target}, config.tags, config.search, config.root)
+		log.Println("===after===")
+		_dump(os.Stdout, []string{tmppath}, config.tags, config.search, config.root)
+	} else if err := rename(tmppath, config.target); err != nil {
+		return err
+	}
+
+	return nil
+}
+
 func archive(config *config) error {
 	if config.archive == "" {
 		return nil
diff --git a/cmd/testdata/1.yaml b/cmd/testdata/1.yaml
index ca66683..f0391e4 100755
--- a/cmd/testdata/1.yaml
+++ b/cmd/testdata/1.yaml
@@ -1,4 +1,18 @@
 todo:
 - "1"
-scheduled: []
-done: []
+- "2"
+- "1"
+- "2"
+- "2"
+scheduled:
+- todo: "10"
+- todo: "20"
+- todo: "10"
+- todo: "20"
+- todo: "20"
+done:
+- todo: "100"
+- todo: "200"
+- todo: "100"
+- todo: "200"
+- todo: "200"
diff --git a/go.mod b/go.mod
index 88ebfa0..dca6add 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module gitea.inhome.blapointe.com/gogs/pttodo
 
-go 1.17
+go 1.21
 
 require (
 	github.com/robfig/cron/v3 v3.0.1