Enabling duplex print on your bundle will insert blank pages into your bundle to ensure it is ready to be printed double-sided.
To enable this, first open the bundle properties by selecting the bundle metadata underneath the bundle title.
This will open up the bundle properties on the right-hand side, where you can toggle duplex print on and off.
If you've already generated your bundle before changing these settings, please regenerate your bundle again for the changes to take effect.