keyboard: bring back uniqueness check

The collision detection code was removed in commit a0a15588
as the cleanup was happening because the previous code was closely
tied to the user interface components.

Because that code wasn't appliable to the new listbox UI, it was
temporarily removed.

This patch re-adds this feature to work with the new code orgazination.

https://bugzilla.gnome.org/show_bug.cgi?id=769063