If you encounter double bookings in your accounting software, it means that you or another user have booked the same days multiple times from Kassatellen.nl.
Each counted day can be booked as many times as you want. However, before you book a day or multiple days, you can always check if they have already been booked. These days have a green box around the date in the calendar. Additionally, when booking, a popup will always appear on your screen indicating that a day or days have already been booked.
Tip
Avoid double bookings by not manually booking days, but by turning on the “auto-booking” function in the settings of your accounting integration (Settings → Integrations).