This is basically a copy of MDL-23273 . The bug is marked as fixed but it is present in 3.1 and 3.2.

To reproduce the bug:
1. Create a new course or use an existing one
2. Add item 'choice' with defaults values excpet the following settings:

Limit the number of responses allowed = enable
some choices where limit = 1
Restrict answering to this time period = yes
Display vertically
Publish results = always show results to students
privacy of results = publish full results, showing names and their choices
group mode = separate groups

3. Login at 2 different PCs with different student account
4. Select same choice and hit 'save my choice' synchronously

As you can see in the attachment, choosing a limited choice is possible. We have received some complaints by teachers using this tool. They report that (somehow) this effect happens quite often, especially when tight time period is enabled.