In most cases, we find that the print server has been installed as a service and additionally opened as a standalone tool.
If you choose to install SKULabs Print and Scale server as a service, you will want to be sure you've not also opened the standalone print and scale tool.
On Windows, the standalone tool would be visible in your task bar or tray area as an orange circle icon. Right click this, then click "Quit".
Alternatively, you can decide to disable the background service by uninstalling then reinstalling without the "Install as background service" checkbox checked.
If you're still having issues with double printing, check your printer settings or contact support by clicking the orange chat icon to your right.