I don't know how they treat 'fortified' wines (Manzanilla), but each adult is allowed one quart of distilled spirits duty free. Once, a relative gave us two huge bottles (magnums?) of Felipe II Brandy the night before we would leave for our return to the U.S. It put us about a gallon over our limit: 'Customs' was nothing, about 40-cents (I breathed a sigh of relief). The IRS Tax was about 12-dollars.

I brought in a case (12 750ml bottles) of Rioja Reserva and they allowed it past customs at JFK.

I love Manzanilla! Another time I waited to buy wines/liquors at the duty free shops in Madrid's aeropuerto de Barajas. They had no Manzanilla! So, my following trip I bought my Manzanilla at a wine shop. Likewise for Albariño (and Rosál). If you want to bring some special wine, don't wait until you get to the airport.