Aquaman and Namor are both Atlantean and Human. Namor debuted first in the April of 1939 and Aquaman debuting 2 years later in November of 1941.
4. The Flash/Quicksilver
The Flash and Quicksilver both have the power of superspeed. The Flash was created long before Quicksilver in January 1940 and Quicksilver debuting nearly 24 years later in March 1964.
3. The Atom/Ant-Man
Both with the ability to grow and shrink themselves and have superstrength while small. The Atom(Ray Palmer) was created in October 1961 and Ant-Man was created a year later in January 1962 and was one of the founding members of The Avengers.
2. Catwoman/Black Cat
Catwoman and Black Cat are both expert burglars and fighters and have pretty much the same costume including the claws. Cat woman was created long before Black Cat in April of 1940 and Black Cat being created 39 years later in July 1979.
Deathstroke and Deadpool are both expert marksmen with a healing factor and have almost the same name, Slade Wilson(Deathstroke) and Wade Wilson(Deadpool). One of the creators of Deadpool liked the Teen Titans comics where Deathstroke debuted. Deathstroke debuted in December 1980 and Deadpool debuted in Febrary 1991.
Is there anything I missed or left out. Let me know in the comments.