duplicate documents shown when loading a font with overlapping document entries

[heycam]

If you have a font with an SVG document index table with multiple entries that point to the same offset/length (e.g. it says glyphs 1-1 have a document at offset/length (123,456) and glyphs 2-2 have a document at offset/length (123,456), then each of these gets a textarea for editing. We should only make one textarea per unique offset/length pair.