After reviewing the email logs, I discovered that my users are receiving Internal Activity Alerts despite being listed as Collaborators on the ticket. They are receiving two emails simultaneously: one from Response/Reply Alert and the other from Internal Activity Alert.
I double-checked my settings, and yet this shouldn't happen because the collaborator is not registered as an agent.
Note that the Ticket Creator does not receive this kind of notification, only the Collaborators do.
your help is appreciated
thank you in advance