tengo un gemelo
gemela
You say, "gemelos."
doble veinte
gemelos - identical twins cuate - twin
you say "que tiene un conjunto de bebés gemelos"
Gemelo/a (Identical twin) Mellizo/a (fraternal twin)
Gemelo.
Doble
"Twin", as a noun, is singular; so, "twin is" is correct. "Twins" or "twin & _____" would be plural, in which case "are" would be correct.
Alma gemela. (pronounced ALL-ma heh-MAY-la)
my twin sisters is mes soeurs jumelles
You would say "孖仔" (maai5 zai2) in Cantonese to refer to twin boys.