That is because, in plural abbreviations, it is very common that each letter is repeated twice. So, EEUU is an example for Estados Unidos, but there are others like CCOO for Comisiones Obreras (a Spanish trade union) or RRHH for Recursos Humanos (HR).
In Spain, people commonly refer to the USA as "Estados Unidos," which translates to "United States" in English. The term "Amrica" is often used to refer to the continent as a whole rather than specifically to the United States.