Same slot by the same member allowed to book multiple times – why?

Hello, we have been using the free version of this plugin for months with no issues. After the latest updates we noticed a lot of double booking. The same person is now allowed to book the same slot multiple times – which was not the case before. Before when x user blocked the 10-12 timeframe it greyed out and he was not able to add that time slot again. Now this changed in the latest updates and now they are allowed to click the same slot again and again – this way creating double bookings. How to fix this? Why did this change?