Part of fixing https://github.com/fonttools/fonttools/issues/2475
Old code was mistakenly only adding the synthetic-generated feature
to the first language-system. In the case of merging Noto Grantha
and Noto Tamil fonts, Noto Tamil had two script systems: 'taml' and
'tml2', and we were adding the synthetic 'locl' feature only to the
DefaultLangSys of the 'taml' script, not the 'tml2' script. That was
the main bug.
A second issue was that the lookup was being added twice to the same
(synthetic) feature. Check before adding lookup, to avoid that.
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