You can only send an invitation once to a single contact. If an invitee's name is in multiple files that are uploaded to the Public Invitation Tool, he/she will only receive the first invitation sent.
If you'd like to send more than one invitation to a contact, please ask your event manager to turn on the Registration Reminder function inside Captix. These emails can be once for each event. They can be sent at any time (with any subject), and are sent to all the invitees of an event who have not yet registered.