glyphsLib should guard against glyphs with duplicate names

Glyphs.app apparently allows multiple glyphs with the same glyph name (which I think it's odd).
Since in UFO the glyph name is unique (it's the key into the contents.plist dict), GlyphsLib should make sure that we don't end up either inadvertently losing data (e.g. a glyph with same name overwriting the other), or with kerning groups in groups.plist with duplicate entries (I was actually bitten by the latter).