You may have a duplicate font that identifies as Helvetica. You can check in Font Book for font issues like corrupted and duplicate fonts. Note that you shouldn't disable "Allow pages to choose their own fonts, instead of your selections above" as this will cause issues with iconic fonts used by webpages to display small icons (you may see text labels instead of icons). You may find that it can be resolved by finding and removing duplicate fonts installed in the local ~/Library/Fonts folder, that are conflicting with standard system fonts.