When copy-pasting the / (slash) from the font I’m creating in Glyphs App to another app I get // (2 slashes). I tried it also with an ‘official’ and existing font, but it has the same effect.
the slash is a bit special as it denotes glyph names. If you copy text form the edit view that contains glyphs without unicode, it puts the glyph name in the string instead. You can see that in the title of the tabs, too. So if you copy text with a slash it adds another slash to make sure that it is a slash and not a glyph name.
Aaah good to know it’s not a bug in my font 
Thanks for the quick reply!