twin
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Gemelli identici are identical twins (from one egg); gemelli fraterni are fraternal (non-identical) twins. 'Gemelli' is also the Italian name for the star sign Gemini, and for cufflinks (from the idea of a matching pair).
Ha un fratello gemello identico a lui.
He has a twin brother identical to him.
I gemelli spesso sviluppano un linguaggio segreto tra loro.
Twins often develop a secret language between themselves.