Why I get duplicated users in iCal

[Guest]

Hi, When my users are editing an event and try to add an inivtation, on the pop-up screen appears both the open directory user and the "mail" user, I mean: Alejandro Fuentes Alejandro Fuentes <alejandro.fuentes@xxxxx.com> Both users are the same but if you select the second one, iCal tries to reach him by mail, I´ve got enable the Enable Mail Invitations option, but I suppouse is used by external guests.