gnome-font-viewer does not check if a font is installed or not before proposing to install it via its install button. (this is true both for system-installed fonts and user-installed fonts). Before showing this button, gnome-font-viewer should query fontconfig to check if this font is not already available (and even if it was not the correct behaviour is to query packagekit to check if it's not already packaged and QAed by the distribution before loading files of unknown origin. Fonts are complex beasts with embedded instructions and not safe to install without checks. Distributions like Fedora have all the required metadata in their font packages i.e. font(fontname) provides)