From left to right: Isaac Newton, Albert Einstein, and Alexander Graham Bell.
Calculus (17th century):
Isaac Newton (left) and Gottfried Wilhelm Leibniz (right) both independently formulated calculus around the same time, among others.
Theory of Evolution (1840, 1857):
Charles Darwin discovered it in 1840, Alfred Russel Wallace separately in 1857. They released a joint publication, 1859.
Polio Vaccine (1950-63):
Hilary Koprowski, Jonas Salk, and Albert Sabin all developed successful polio vaccines independent of each other.
While Einstein is credited for producing the famous equation with his theory of relativity, Henri Poincaré, Olinto De Pretto, and Paul Langevin all came up with similar theories of the mass-energy relationship.
Carl Wilhelm Scheele (1773), Joseph Priestley (1774), and Antoine Lavoisier (1777) all discovered oxygen on their own -- but it was Lavoisier who coined the term.
Orville and Wilbur Wright (pictured) soared into history books on Dec. 17, 1903, following their historic, 852-foot, 59-second flight at Kitty Hawk, North Carolina -- an achievement for which the duo are widely described as the first in flight. But the first flight may have happened two years before that, when German immigrant Gustav Whitehead, born Weisskopf -- flew a winged, bird-like plane called #21 on the morning of Aug. 14, 1901.
Elisha Gray and Alexander Graham Bell filed a patent on discovery of the telephone on the same day.
Jet Engine (1939-1941):
It was independently invented by and used in working aircraft by Hans von Ohain (1939), Secondo Campini (1940) and Frank Whittle (1941). The Heinkel He 178 (pictured) was the world's first aircraft to fly purely on turbojet power.
Higgs Boson (1964):
The "God Particle," as it's sometimes known (to the chagrin of scientists everywhere), was developed into a full relativistic model in 1964 independently and almost simultaneously by three groups of physicists: François Englert and Robert Brout; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble.
Throughout history, major scientific breakthroughs and notable inventions have occurred simultaneously and independently among different thinkers and inventors, who, more often than not, had no direct contact with each other. The phenomenon is known as "multiple discovery." The only problem? Who gets credit? In the case of "the first flight," the argument continues.