Find duplicates even when the full user reference is not entered in the user picker

Right now, when using the user picker and allowing to enter several users, if you type a user name (without the XWiki. prefix and also select that user from the list, it's going to be considered a duplicate down the line the input will be used for performing some actions (unless the code checks for dups again). It would be better to have the user picker resolve the references before comparing them for duplicates.