If the same customer is the owner of two domains under the same Webex org and end-users move from 1 domain to the other, the field "Attendees" will continue to suggest the end-user with the old domain in suggestion, in this case, you see the same end-user but twice, 1 with the new domain and 1 with the old domain. This field suggestion is not cleaned up by design by Cisco, according to them, this is working like this.
The direction of scheduling meetings is from the Webex app scheduler feature where there is only 1 set of users from Webex Identity. There are no duplicates by design. If a user moves between Webex Sites or their email address changes for whatever reason, then it will only be 1 user—there will not be duplicates.