diff --git a/lib/travis/api/app/endpoint/users.rb b/lib/travis/api/app/endpoint/users.rb
index c6067b90..ff109c2c 100644
--- a/lib/travis/api/app/endpoint/users.rb
+++ b/lib/travis/api/app/endpoint/users.rb
@@ -47,7 +47,12 @@ class Travis::Api::App
       end
 
       post '/sync', scope: :private do
-        respond_with service(:sync_user)
+        if current_user.syncing?
+          status 409
+          { 'message' => "Sync already in progress. Try again later." }
+        else
+          respond_with service(:sync_user)
+        end
       end
     end
   end
diff --git a/spec/unit/endpoint/users_spec.rb b/spec/unit/endpoint/users_spec.rb
index 7675b170..46e0ff7e 100644
--- a/spec/unit/endpoint/users_spec.rb
+++ b/spec/unit/endpoint/users_spec.rb
@@ -29,4 +29,19 @@ describe Travis::Api::App::Endpoint::Users do
       'correct_scopes' => true,
     }
   end
+
+  context 'when responding to POST for /users/sync' do
+    context 'when sync is in progress' do
+      before :each do
+        user.stubs(:syncing?).returns(true)
+      end
+
+      it 'returns 409' do
+        response = post('/users/sync', { access_token: access_token.to_s }, 'HTTP_ACCEPT' => 'application/vnd.travis-ci.2+json, */*; q=0.01')
+
+        response.status.should == 409
+        JSON.parse(response.body).should be_true
+      end
+    end
+  end
 end