TOPIC: Selection limit for dropdown doesn't work with groups

I am using DTRegister 2.6.8 and Selection limit for dropdown doesn't work with groups registrations.
In our case we have offered the option to register for a physiotherapeutic massage and we have entered 10 possible slots per day 30 minutes each. The slot is for one person only.
When I now register a group, every person can chose the same slots in the dropdown altough there is limit set to 1.
As every registration happens on a single page, the algorithm should consider this scenario and should remove the slot from the dropdown list so that next person in the group registration cannot select the same slot again.

How can I achive this ?