Olinto De Pretto published E=mc^2 in 1903.
This was 2 years before Einstein "discovered" it.
In 1903, the Italian Olinto De Pretto, who was an engineer/industrialist with experience in materials and their properties, gave the precise formula E = mc2. It was first published in June 1903. De Pretto delivered a second paper on November 29th 1903 in Venice, and this paper was published in the proceedings of the Venetian Royal Institute of Science, Literature and Art in February 1904.
O. De Pretto, "Ipostesi dell'etere nella vita dell'universo", Atti del Reale Istituto Veneto di Scienze, Lettere ed Arti, Volume 63, Part 2, (February, 1904), pp. 439-500.
It is fairly certain that Einstein knew of De Pretto's work.
Olinto De Pretto stated:
Given then E = mc^2, m = 1 kg and c = 3 x 10^5 km/s. anyone can see that the quantity of calories obtained is represented by 16794 followed by 9 zeros, that is more than ten thousand billions. To what terrible result has our reasoning brought us? Nobody will easily admit that an amount of energy equal to the quantity that can be derived from millions and millions of kilograms of coal is concealed and stored at a latent state in one kilogram of matter of any kind this idea will be undoubtedly considered foolish. However, even if the result of our calculations be reduced somewhat, it should be nevertheless admitted that inside matter there must be stored so much energy as to strike anyone's imagination. What is in comparison to it, the energy that can be derived from the richest combustible or from the most powerful chemical reaction?
Also E=mc^2 was implicit in Poincaré's 1900 paper:
H. Poincaré, "La Théorie de Lorentz at le Principe de Réaction", Archives Néerlandaises des Sciences Exactes et Naturelles, Series 2, Volume 5, Recueil de travaux offerts par les auteurs à H. A. Lorentz, professeur de physique à l'université de Leiden, à l'occasion du 25 me anniversaire de son doctorate le 11 décembre 1900, Nijhoff, The Hague, (1900), pp. 252-278; reprinted Œuvres, Volume IX, p. 464-488.
This was 5 years before Einstein "discovered" it.
It is certain that Einstein knew of this work.
Indeed, in a paper in 1906, Einstein acknowledged that Poincaré had already derived the equivalence (i.e., e=mc^2). When commenting on his own 1905 paper, where he originally gave the equivalence. Einstein wrote, "Even though the simple formal observations which must lead to the proof of this assumption is already contained in the main in a work by H Poincaré, I, for reasons of clarity, will not refer to that particular work" One wonders what extra 'clarity' resulted from Einstein not quoting from the earlier work of Poincaré.
Apparently, Fritz Hasenöhrl also published E = mc^2 before Einstein "discovered" it.
Throughout his career Einstein was (justly) accused of plagiarizing the result E = mc^2.
