We have to think about what we have to do about name uniqueness if the option for using the real_name as the display name is in use. Real names are not unique, so different users can end up with the same display name. I have no problems with that for my own site, but I can see uniqueness of names on a site as a requirement for others.
On important thing here is that simply making the real names unique won't do the trick. Because the display name will be the username if no real name is available. So we might need to add some checks which check both the username and real_name when changing/creating one of those for a user.
Reported by: mmakaay
Imported from TRAC: http://trac.phorum.org/ticket/585
IMO, if you allow real names to be displayed, you must live with the fact that people in the world do have the same name from time to time. It just needs to be a warning and the admins will have to live with it.
I'm one of these strange people which require the real-name to be unique but I can also do this through a module. no problem with that.
Sorry, that was me that made that first comment.
I found a case where we would need some additional support somehow for non-unique display names. When adding groups or sending PM by typing in the user's name, the display name might be used by multiple users. So, searching for a name does not yield a unique result and makes it impossible to add the user to the recipients or groups.
I think the cleanest way of implementing this properly, would be to allow for a list of users (including a link to their profile) to select from in case multiple users did match.
Moved to 5.3.0. No user reports till now and because this might require template changes, we'd better look at it for 5.3.0 (but we should beware that the 5.2 template will still be forward compatible, because we decided that 5.2 templates should work in 5.3).
Revision  resolves this for PMs somewhat. The user at least knows why the add recip failed. If they want to find the person, they will. Not the best solution, but short of a user browser, not sure what else we can do.