Duplicate glyph in class

This works in feaLib and makeotf:

@foo = [A A];
feature kern { pos @foo 10; } kern;

This works in makeotf (with a warning) but errors in feaLib:

@foo = [A A];
feature kern { pos @foo 10 @foo; } kern;

Admittedly it’s bad code, (harder for a user to spot when there are thirty other glyphs between the first and last occurrence) but there is something sensible that can be done with it, so we should.

[justvanrossum]

Yes, at the .fea level, duplicates are allowed and useful, even for single substitutions:

feature ss01 {
	sub [a b c] by [Z Z Z];
} ss01;

[BoldMonday]

What about cases of many-to-many substitutions where the length of both classes needs to be the same?

If a duplicate glyph is ignored you might get a compilation error that doesn’t really point to the actual problem.