diff --git a/app/Console/Commands/DedupTags.php b/app/Console/Commands/DedupTags.php
new file mode 100644
index 0000000..3300e59
--- /dev/null
+++ b/app/Console/Commands/DedupTags.php
@@ -0,0 +1,113 @@
+<?php
+
+namespace App\Console\Commands;
+
+use App\Tag;
+use Illuminate\Console\Command;
+use Illuminate\Support\Facades\DB;
+
+class DedupTags extends Command
+{
+    /**
+     * The name and signature of the console command.
+     *
+     * @var string
+     */
+    protected $signature = 'tissue:tag:dedup {--dry-run}';
+
+    /**
+     * The console command description.
+     *
+     * @var string
+     */
+    protected $description = 'Deduplicate tags';
+
+    /**
+     * Create a new command instance.
+     *
+     * @return void
+     */
+    public function __construct()
+    {
+        parent::__construct();
+    }
+
+    /**
+     * Execute the console command.
+     *
+     * @return mixed
+     */
+    public function handle()
+    {
+        if ($this->option('dry-run')) {
+            $this->warn('dry-runモードで実行します。');
+        } else {
+            if (!$this->confirm('dry-runオプションが付いてないけど、本当に実行しますか？')) {
+                return;
+            }
+        }
+
+        DB::transaction(function () {
+            $duplicatedTags = DB::table('tags')
+                ->select('name', DB::raw('count(*)'))
+                ->groupBy('name')
+                ->having(DB::raw('count(*)'), '>=', 2)
+                ->get();
+
+            $this->info($duplicatedTags->count() . ' duplicated tags found.');
+
+            foreach ($duplicatedTags as $tag) {
+                $this->line('Tag name: ' . $tag->name);
+
+                $tagIds = Tag::where('name', $tag->name)->orderBy('id')->pluck('id');
+                $newId = $tagIds->first();
+                $dropIds = $tagIds->slice(1);
+
+                $this->line('  New ID: ' . $newId);
+                $this->line('  Drop IDs: ' . $dropIds->implode(', '));
+
+                if ($this->option('dry-run')) {
+                    continue;
+                }
+
+                // 同じタグ名でIDが違うものについて、全て統一する
+                foreach (['ejaculation_tag', 'metadata_tag'] as $table) {
+                    DB::table($table)
+                        ->whereIn('tag_id', $dropIds)
+                        ->update(['tag_id' => $newId]);
+                }
+                DB::table('tags')->whereIn('id', $dropIds)->delete();
+
+                // 統一した上で、重複しているレコードを削除する
+                DB::delete(
+                    <<<SQL
+DELETE FROM ejaculation_tag
+WHERE id IN (
+    SELECT id
+    FROM (
+        SELECT id, row_number() OVER (PARTITION BY ejaculation_id, tag_id ORDER BY id) AS ord
+        FROM ejaculation_tag
+    ) t
+    WHERE ord > 1
+)
+SQL
+                );
+                DB::delete(
+                    <<<SQL
+DELETE FROM metadata_tag
+WHERE id IN (
+    SELECT id
+    FROM (
+        SELECT id, row_number() OVER (PARTITION BY metadata_url, tag_id ORDER BY id) AS ord
+        FROM metadata_tag
+    ) t
+    WHERE ord > 1
+)
+SQL
+                );
+            }
+        });
+
+        $this->info('Done!');
+    }
+}
diff --git a/database/migrations/2020_02_22_164304_add_unique_constraint_to_tag_relations.php b/database/migrations/2020_02_22_164304_add_unique_constraint_to_tag_relations.php
new file mode 100644
index 0000000..b8d6f50
--- /dev/null
+++ b/database/migrations/2020_02_22_164304_add_unique_constraint_to_tag_relations.php
@@ -0,0 +1,38 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class AddUniqueConstraintToTagRelations extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up()
+    {
+        Schema::table('ejaculation_tag', function (Blueprint $table) {
+            $table->unique(['ejaculation_id', 'tag_id']);
+        });
+        Schema::table('metadata_tag', function (Blueprint $table) {
+            $table->unique(['metadata_url', 'tag_id']);
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     *
+     * @return void
+     */
+    public function down()
+    {
+        Schema::table('ejaculation_tag', function (Blueprint $table) {
+            $table->dropUnique(['ejaculation_id', 'tag_id']);
+        });
+        Schema::table('metadata_tag', function (Blueprint $table) {
+            $table->dropUnique(['metadata_url', 'tag_id']);
+        });
+    }
+}