Luise Rainer
(The Great Ziegfeld and The Good Earth)
Vivien Leigh
(Gone With the Wind and A Streetcar Named Desire)
Hillary Swank
(Boys Don't Cry and Million Dollar Baby)

Helen Hayes also won two acting awards on her only nominations, Best Actress for The Sin of Madelon Claudet in 1931/32 and Best Supporting Actress for Airport in 1970.

Sally Field won Best Actress for her only two entries in that category (Norma Rae in 1979 and Places in the Heart in 1986), but was nominated for Best Supporting Actress for Lincoln in 2012. 

The only actors to go two for two at the Oscars are Kevin Spacey, who won Best Supporting Actor for The Usual Suspects (1995) and Best Actor for American Beauty (1999), Christoph Waltz, who won Best Supporting Actor for Inglorious Basterds (2010) and Django Unchained (2012), and Mahershala Ali, who win Best Supporting Actor for Moonlight (2016) and Green Book (2018)..