If a user is invited to an appointment, which conflicts with another appointment, no warning is shown. So it can happen the user accepts this appointment, without noticing the conflict at all, which can lead to problematic situations. At least a warning in the same style as the inviting person is shown when trying to invite another person, who already has got an appointment at that time. Most probably this will happen, when E-Mails with invitations arrive that are not sent from inside the same SOGo instance or SOGo at all.