Abbreviation of los Estados Unidos
Why is los Estados Unidos abbreviated as EE UU ?
The double E and double U indicate that the letter represents a plural. Some other common Spanish abbreviations, among them FF. AA. for Fuerzas Armadas (Armed Forces) and AA. EE. for Asuntos Exteriores (Foreign Affairs), do the same thing. (Also in very common use are the abbreviations without the spaces and/or periods, such as EEUU, FFAA and AAEE.) Such a doubling of letters isn't done for all plurals; ONU is the abbreviation for la Organización de las Naciones Unidas, the United Nations.
Based in Latin, plural abbreviations' letters are doubled in English, Spanish, and possibly other languages. Like the plural word "page" is abbreviated as "p." and "pages" is abbreviated as "pp.", "Estado Unido" (United State) is abbreviated as "E.U." and Estados Unidos (United States) is abbreviated as "EE.UU."
Because states are in the plural, the European Union on the other hand is singular La unión de Europa so it is abreviated to UE.