Different warnings message for sets of same content different name vs same content same name

[unhammer]

These two errors get the same warning:

LIST foo = foo;
LIST foo = foo;
LIST bar = bar;
LIST baz = bar;

but it would be nice if they differed, since the first one is a trivial thing which should always be fixed, while sometimes we want to ignore the second warning (e.g. grammars for different languages might have synchronised set names for easy diffing and copy-pasting, and in one of the languages a set happens to duplicate the contents of another)