Normalizer can decompose/compose your character as you like:
String decomposed = Normalizer.normalize(String.valueOf('?'), Form.NFD);
result are two character (i, double-dot)
String composed = Normalizer.normalize(decomposed, Form.NFC);
result is one character (?)
If I understand you correctly you seek
return Normalizer.normalize("u0069u0307", Form.NFC).charAt(0);
For double dots use u0308
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