Glyph classes from feaLib parser are tuples of strings, with an order and possibly containing duplicates. However when building MarkGlyphSetsDef or MarkAttachClassDef we are only interested in the *set* of glyphs they contain, i.e. the unordered collection of unique glyph names. Also, note how in the tests for otlLib.builder.buildMarkGlyphSetsDef, the input is given as set literals, not tuples: https://github.com/fonttools/fonttools/blob/78ad48e/Tests/otlLib/builder_test.py#L633