diff --git a/app/controllers/discourse_assign/assign_controller.rb b/app/controllers/discourse_assign/assign_controller.rb
index 2753480..32250bd 100644
--- a/app/controllers/discourse_assign/assign_controller.rb
+++ b/app/controllers/discourse_assign/assign_controller.rb
@@ -62,6 +62,10 @@ module DiscourseAssign
 
       raise Discourse::NotFound unless assign_to
 
+      if topic.custom_fields && topic.custom_fields['assigned_to_id'] == assign_to.id.to_s
+        return render json: { failed: 'Already assigned to the user' }, status: 400
+      end
+
       assigner = TopicAssigner.new(topic, current_user)
 
       # perhaps?
diff --git a/spec/requests/assign_controller_spec.rb b/spec/requests/assign_controller_spec.rb
new file mode 100644
index 0000000..1b63310
--- /dev/null
+++ b/spec/requests/assign_controller_spec.rb
@@ -0,0 +1,39 @@
+require 'rails_helper'
+
+RSpec.describe DiscourseAssign::AssignController do
+
+  let(:user) { Fabricate(:admin) }
+  let(:post) { Fabricate(:post) }
+  let(:user2) { Fabricate(:active_user) }
+
+  context 'assign' do
+
+    it 'assigns topic to a user' do
+      sign_in(user)
+
+      put '/assign/assign', params: {
+        topic_id: post.topic_id, username: user2.username
+      }
+
+      expect(response.status).to eq(200)
+    end
+
+    it 'fails to assign topic to the user if its already assigned to the same user' do
+      sign_in(user)
+
+      put '/assign/assign.json', params: {
+        topic_id: post.topic_id, username: user2.username
+      }
+
+      expect(response.status).to eq(200)
+
+      put '/assign/assign.json', params: {
+        topic_id: post.topic_id, username: user2.username
+      }
+
+      expect(response.status).to eq(400)
+    end
+
+  end
+
+end