"Estados Unidos" is abbreviated as "EEUU" (also: EE. UU.) because in
Spanish the abbreviation for a plural item doubles the letter of the
abbreviation. The same is done, for example, with the abbreviation for
"Fuerzas Armadas" (= Armed Forces), which is "FF. AA.", indicating
that the abbreviation relates to a plural term.
The English language knows this custom too, albeit in a few cases
only. For instance, the abbreviation for "page" in singular is "p.",
while "pages" in plural are abbreviated as "pp.".
Sometimes, however, the abbreviation "EUA" (Estados Unidos de América)
for the United States is indeed used in Spanish, but not often.
About.com: Spanish Language - Why EE. UU. for Estados Unidos?
Dept. of Modern Foreign Languages, Indiana University - Purdue
University Fort Wayne: Lesson 8 - More on Articles, by Daniel
Search terms uses:
eeuu estados abbreviation
eeuu estados abbreviation plural
Hope this answers your question!