diff --git a/lib/sprockets/railtie.rb b/lib/sprockets/railtie.rb
index 3ff11003..8e63a1f7 100644
--- a/lib/sprockets/railtie.rb
+++ b/lib/sprockets/railtie.rb
@@ -103,9 +103,9 @@ def configure(&block)
     initializer :set_default_precompile do |app|
       if using_sprockets4?
         raise ManifestNeededError unless ::Rails.root.join("app/assets/config/manifest.js").exist?
-        app.config.assets.precompile += %w( manifest.js )
+        app.config.assets.precompile = %w( manifest.js )
       else
-        app.config.assets.precompile += [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
+        app.config.assets.precompile = [LOOSE_APP_ASSETS, /(?:\/|\\|\A)application\.(css|js)$/]
       end
     end