In 1904, the year before Einstein’s seminal papers on special relativity, Austrian physicist Fritz Hasenöhrl examined the properties of blackbody radiation in a moving cavity. He calculated the work necessary to keep the cavity moving at a constant velocity as it fills with radiation and concluded that the radiation energy has associated with it an apparent mass such that E = mc2. In a subsequent paper, also in 1904, Hasenöhrl achieved the same result by computing the force necessary to accelerate a cavity already filled with radiation. In early 1905, he corrected the latter result to E = mc2.These papers were published in the same journals that Einstein published in. Einstein went his entire life refusing to acknowledge or credit this work.
There was also earlier work by Poincare in 1900.
This is not an isolated example. Einstein's most famous papers refused to reference the earlier work, even when he obviously relied on that earlier work. Throughout his life, he always took credit for himself, at the expense of others, wheneven he could get away with it. And also when everyone knew he was dishonest, as in the above example.
I don't mind him taking legitimate credit for himself. But the way to do that is for him to explain how his contribution improves on earlier work. He was never able to do that with relativity theory.