Maybe filter out duplicate custom parameters and ensure master names are unique

Stumbled over a .glyphs file that had two custom parameters for "Master Name" that were the same. Some masters had the same name even, which lead to TTFs overwriting each other or something when doing `fontmake -m something.designspace -i -o variable`.

Start by tracing how glyphsLib reads custom parameters and master names from a .glyphs file, then reproduce the problem with the fontmake command described in the issue. Done means duplicate "Master Name" parameters are filtered and master names remain unique so generated TTFs do not overwrite one another.

Written by the indexing model from the issue text.