A new enhancement on the My Disney Experience website now helps in preventing duplicate managed Guests. If someone attempts to add a new Guest manually by typing their name or by using the "People You May Know," option and the name is a duplicate of someone already on their Family & Friends list, the site will alert the Guest there is a name match. The Guest will then be asked if they still wish to add the Guest.
Here you are some screenshots showing the change, which will definitely help Guests in the future:
This functionality only appears in the "Add a Guest" flow under Family & Friends. The messaging does not appear if a new Guest is added while purchasing tickets or making/linking hotel and dining reservations.
Additionally, this functionality is only available on the web site and through a mobile web browser, but is not available in the My Disney Experience app.
SCREENSHOTS: ©2015 Disney. All Rights Reserved.