diff --git a/app/controllers/microposts_controller.rb b/app/controllers/microposts_controller.rb
index c87e501..f0a2fce 100644
--- a/app/controllers/microposts_controller.rb
+++ b/app/controllers/microposts_controller.rb
@@ -17,7 +17,7 @@ class MicropostsController < ApplicationController
   def create
     micropost = Micropost.new(micropost_params)
     micropost.user = helpers.current_user
-    micropost.add_tags(params[:tags])
+    micropost.set_tags(params[:tags]) if params[:tags]
     if micropost.save
       redirect_to micropost, notice: t(".created")
     else
@@ -30,6 +30,7 @@ class MicropostsController < ApplicationController
   def edit; end
 
   def update
+    @micropost.set_tags(params[:tags]) if params[:tags]
     if @micropost.update(micropost_params)
       redirect_to @micropost, notice: t(".updated")
     else
diff --git a/app/helpers/microposts_helper.rb b/app/helpers/microposts_helper.rb
new file mode 100644
index 0000000..87e3631
--- /dev/null
+++ b/app/helpers/microposts_helper.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module MicropostsHelper
+  def tag_string(micropost)
+    micropost.tags.map(&:name).join(" ")
+  end
+end
diff --git a/app/models/micropost.rb b/app/models/micropost.rb
index 4d97031..d255249 100644
--- a/app/models/micropost.rb
+++ b/app/models/micropost.rb
@@ -11,9 +11,10 @@ class Micropost < ApplicationRecord
   has_many :microposts_tags
   has_many :tags, through: :microposts_tags
 
-  def add_tags(*tag_names)
-    tag_names.each do |tag|
-      tags << Tag.find_or_create_by(name: tag)
+  def set_tags(*tag_names)
+    tag_names.each do |tag_name|
+      tag = Tag.find_or_create_by(name: tag_name)
+      tags << tag unless tags.include?(tag)
     end
   end
 end
diff --git a/app/views/microposts/_form.html.erb b/app/views/microposts/_form.html.erb
index 476dfdf..d3653ea 100644
--- a/app/views/microposts/_form.html.erb
+++ b/app/views/microposts/_form.html.erb
@@ -3,7 +3,7 @@
 
   <%= f.rich_text_area :content %>
 
-  <%= text_field_tag :tags %>
+  <%= text_field_tag :tags, tag_string(@micropost) %>
 
   <%= f.submit button_text %>
 <% end %>
diff --git a/test/models/micropost_test.rb b/test/models/micropost_test.rb
index 8235133..40167aa 100644
--- a/test/models/micropost_test.rb
+++ b/test/models/micropost_test.rb
@@ -21,7 +21,17 @@ class MicropostTest < ActiveSupport::TestCase
   test "can add tags" do
     assert_empty @micropost.tags
     assert_difference "@micropost.tags.count", +3 do
-      @micropost.add_tags("foo", "bar", "baz")
+      @micropost.set_tags("foo", "bar", "baz")
+    end
+  end
+
+  test "duplicate tags are not added" do
+    assert_empty @micropost.tags
+    assert_difference "@micropost.tags.count", +3 do
+      @micropost.set_tags("foo", "bar", "baz")
+    end
+    assert_no_difference "@micropost.tags.count" do
+      @micropost.set_tags("bar", "baz")
     end
   end
 end