thanks for the feedback. Bookings does not support double booking towards the same staff today. In fact, that is one of the things that Bookings tries to prevent by design. Here are a few workarounds you can try:
1. Have multiple training users created so that even if one is already booked, users can still book time against the other. Internally, you can then cancel/reschedule/reassign these appointments as required based on trainer availability.
2. If the trainer will be attending to multiple people at the same time, you can create a group service by setting "Maximum number of attendees" to more than 1 so that you can collect more than one signup for the same slot. Note that this field is available only while creating a service.
3. Checkout the Virtual Appointments app if you want to get some sort of a queuing mechanism. Note that this requires you to get a Teams Premium license: https://learn.microsoft.com/en-us/microsoftteams/manage-virtual-appointments-app