Duplicate fonts is a problem in OSX (and has been a problem on Macs in general since they first came out). Open Font Book (in your Applications folder) and search for Junegull. If more than one copy shows up, disable or remove one of the copies. You should be able to remove all copies from Font Book because Junegull is actually originally installed in FCPX. (The path to the fonts installed in FCPX is:
Final Cut Pro.app/Contents/Frameworks/Flexo.framework/Versions/A/Resources/Fonts/
[right click on Final Cut Pro in Applications and select Show Contents]
)
If you only have one copy of Junegull in Font Book, validate the font to see if it has become corrupted in any way. If it has, you will have to remove it. If the version in FCPX is corrupt, it should be replaced. To do that you will have to reinstall the application (I cannot recommend switching out/removing and/or adding embedded resources within an application).
You may install one copy of any of the FCPX fonts in Font Book if you want to be able to use that font in other applications.