During a survey taken on the evaluation tool we came across the error that when we had a large number of participants and we updated the participant list to reflect changes in the class lists, this created duplicate entries of participants in the list.

It happens both for adhoc and Sakai users and it seems to be caused by multi-clicking (although the button gets disabled when you click on it). It's related with long lists of users, but we've also reproduced it with tens instead of thousands.