I'm trying to send an appointment invitation to multiply user(addresses).When one of the user already got a full day event in he's/she's calendar, the invitation don't even show in the user's inbox and that user will not know about the appointment.

But when an user just got some stray appointments that day the invitation will appear in the user's inbox and she/he has the chance to accept/decline the appointment. Is this a known issue? Have i missed something?